Allow Task.Source/Task.Sources to be constructed directly from subpath string literals (#4486)

First step in #4447, by
providing an alternative to the previous `os.Path` APIs.

Effectively this allows us to replace

```scala
def mainScript = Task.Source { millSourcePath / "src/foo.py" }
```

with

```scala
def mainScript = Task.Source { "src/foo.py" }
```

Pulls in com-lihaoyi/os-lib#353 from upstream to
make constructing `os.SubPath`s more ergonomic by eliding the lead
`os.sub /` prefix in the case of literal strings while still maintaining
a degree of safety:

* "outer" paths starting with `..`s and absolute paths starting with `/`
are rejected at compile time
* Only literal strings are converted implicitly, anything non-literal
needs to be an explicit `os.SubPath`


For now we provide this as an alternative to passing in an absolute
`os.Path`, but probably 99% of scenarios should be using this sub-path
API rather than absolute paths since (a) it's more concise and (b) your
sources should be within your `millSourcePath` anyway. I'm not sure we
can get rid of the `os.Path`-taking API entirely, but we can definitely
de-prioritize it and call it `SourcesUnsafe` or something so that anyone
who needs it can use it but most people won't use it accidentally

Author: lihaoyi

build.mill

@@ -158,7 +158,7 @@ object Deps {
   val junitInterface = ivy"com.github.sbt:junit-interface:0.13.3"
   val commonsIo = ivy"commons-io:commons-io:2.18.0"
   val log4j2Core = ivy"org.apache.logging.log4j:log4j-core:2.24.3"
-  val osLib = ivy"com.lihaoyi::os-lib:0.11.4-M5"
+  val osLib = ivy"com.lihaoyi::os-lib:0.11.4-M6"
   val pprint = ivy"com.lihaoyi::pprint:0.9.0"
   val mainargs = ivy"com.lihaoyi::mainargs:0.7.6"
   val millModuledefsVersion = "0.11.3-M3"
@@ -398,7 +398,7 @@ trait MillJavaModule extends JavaModule {
 
   def writeLocalTestOverrides = Task.Anon {
     for ((k, v) <- testTransitiveDeps()) {
-      os.write(Task.dest / "mill" / "local-test-overrides" / k, v, createFolders = true)
+      os.write(Task.dest / "mill/local-test-overrides" / k, v, createFolders = true)
     }
     Seq(PathRef(Task.dest))
   }

contrib/playlib/src/mill/playlib/Layout.scala

@@ -5,8 +5,8 @@ import mill.scalalib._
 
 private[playlib] trait Layout extends JavaModule {
 
-  def conf = Task.Sources { millSourcePath / "conf" }
-  def app = Task.Sources { millSourcePath / "app" }
+  def conf = Task.Sources { "conf" }
+  def app = Task.Sources { "app" }
 
   override def sources = Task { app() }
   override def resources = Task { conf() }

contrib/playlib/src/mill/playlib/RouterModule.scala

@@ -9,7 +9,7 @@ import mill.{Agg, T, Task}
 
 trait RouterModule extends ScalaModule with Version {
 
-  def routes: T[Seq[PathRef]] = Task.Sources { millSourcePath / "routes" }
+  def routes: T[Seq[PathRef]] = Task.Sources { "routes" }
 
   def routeFiles = Task {
     val paths = routes().flatMap(file => os.walk(file.path))

contrib/playlib/src/mill/playlib/Static.scala

@@ -25,7 +25,7 @@ trait Static extends ScalaModule {
   /**
    *  Directories to include assets from
    */
-  def assetSources = Task.Sources { millSourcePath / assetsPath() }
+  def assetSources = Task.Sources { os.sub / assetsPath() }
 
   /*
   Collected static assets for the project

contrib/versionfile/src/mill/contrib/versionfile/VersionFileModule.scala

@@ -5,7 +5,7 @@ import mill._
 trait VersionFileModule extends Module {
 
   /** The file containing the current version. */
-  def versionFile: T[PathRef] = Task.Source(millSourcePath / "version")
+  def versionFile: T[PathRef] = Task.Source("version")
 
   /** The current version. */
   def currentVersion: T[Version] = Task { Version.of(os.read(versionFile().path).trim) }

dist/package.mill

@@ -41,7 +41,7 @@ trait InstallModule extends build.MillPublishJavaModule {
   def installLocalCache() = Task.Command {
     val path = installLocalTask(
       Task.Anon(
-        (os.home / ".cache" / "mill" / "download" / (build.millVersion() + batExt)).toString()
+        (os.home / ".cache/mill/download" / (build.millVersion() + batExt)).toString()
       )
     )()
     Task.log.outputStream.println(path.toString())

docs/modules/ROOT/pages/comparisons/maven.adoc

@@ -391,7 +391,7 @@ import $ivy.`ant:ant-optional:1.5.3-1`
 
 object common extends NettyModule{
   ...
-  def script = Task.Source(millSourcePath / "src" / "main" / "script")
+  def script = Task.Source("src" / "main" / "script")
   def generatedSources0 = Task {
     val shell = new groovy.lang.GroovyShell()
     val context = new java.util.HashMap[String, Object]
@@ -520,8 +520,8 @@ with the `make` command essentially being a bash script wrapped in layers of XML
 In contrast, the Mill configuration for this logic is as follows:
 
 ```scala
-def makefile = Task.Source(millSourcePath / "Makefile")
-def cSources = Task.Source(millSourcePath / "src" / "main" / "c")
+def makefile = Task.Source("Makefile")
+def cSources = Task.Source("src" / "main" / "c")
 def cHeaders = Task {
   for(p <- os.walk(cSources().path) if p.ext == "h"){
     os.copy(p, Task.dest / p.relativeTo(cSources().path), createFolders = true)

docs/modules/ROOT/pages/comparisons/unique.adoc

@@ -176,8 +176,8 @@ them into classfiles, and then the `jar` executable to package them together int
 ```scala
 def mainClass: T[Option[String]] = Some("foo.Foo")
 
-def sources = Task.Source(millSourcePath / "src")
-def resources = Task.Source(millSourcePath / "resources")
+def sources = Task.Source("src")
+def resources = Task.Source("resources")
 
 def compile = Task {
   val allSources = os.walk(sources().path)

docs/package.mill

@@ -68,16 +68,16 @@ object `package` extends RootModule {
       envArgs = Map("CI" -> "true"),
       workingDir = workDir
     )
-    PathRef(workDir / "build" / "site")
+    PathRef(workDir / "build/site")
   }
 
   def source0 = Task.Source(millSourcePath)
   def projectChangelog = Task.Source(Task.workspace / "changelog.adoc")
   def source = Task {
     os.copy(source0().path, Task.dest, mergeFolders = true)
 
-    val pagesWd = Task.dest / "modules" / "ROOT" / "pages"
-    val partialsWd = Task.dest / "modules" / "ROOT" / "partials"
+    val pagesWd = Task.dest / "modules/ROOT/pages"
+    val partialsWd = Task.dest / "modules/ROOT/partials"
 
     os.copy(projectChangelog().path, partialsWd / "project-changelog.adoc", createFolders = true)
 
@@ -291,7 +291,7 @@ object `package` extends RootModule {
       val checkout = Task.dest / displayVersion
       os.proc("git", "clone", Task.workspace / ".git", checkout).call(stdout = os.Inherit)
       os.proc("git", "checkout", millLastTag).call(cwd = checkout, stdout = os.Inherit)
-      val outputFolder = checkout / "out" / "docs" / "source.dest"
+      val outputFolder = checkout / "out/docs/source.dest"
       os.proc("./mill", "-i", "docs.source").call(cwd = checkout, stdout = os.Inherit)
       expandDiagramsInDirectoryAdocFile(
         outputFolder,
@@ -367,7 +367,7 @@ object `package` extends RootModule {
     // only copy the "api" sub-dir; api docs contains a top-level index.html with we don't want
     val unidocSrc = if (authorMode) site.unidocLocal().path else site.unidocSite().path
     Task.log.errorStream.println(s"Copying API docs from ${unidocSrc} ...")
-    os.copy(unidocSrc, siteDir / "api" / "latest", createFolders = true)
+    os.copy(unidocSrc, siteDir / "api/latest", createFolders = true)
 
     PathRef(siteDir)
   }

example/extending/jvmcode/1-subprocess/build.mill

@@ -22,7 +22,7 @@ object foo extends JavaModule {
     defaultResolver().resolveDeps(Agg(ivy"org.codehaus.groovy:groovy:3.0.9"))
   }
 
-  def groovyScript = Task.Source(millSourcePath / "generate.groovy")
+  def groovyScript = Task.Source("generate.groovy")
 
   def groovyGeneratedResources = Task {
     Jvm.runSubprocess(