Remove requirement to extends RootModule (#5008)

Fixes #4971

With this PR, any top-level `object package` in a `build.mill` or
`package.mill` file has the `extends RootModule` or `extends
SubfolderModule` automatically inserted.

Before this PR we were already over-writing the `extends RootModule`
with the correct type, so this just makes it insert the correct type
even when `extends RootModule` is not provided (although we still
require that it extends some kind of `mill.Module`).

If there is an existing `extends RootModule`, we accept that for now to
ease in the migration

Author: lihaoyi

example/extending/metabuild/4-meta-build/build.mill

@@ -3,7 +3,7 @@ package build
 import mill._, scalalib._
 import scalatags.Text.all._
 
-object `package` extends RootModule with ScalaModule {
+object `package` extends ScalaModule {
   def scalaVersion = "2.13.4"
   def mvnDeps = Seq(
     mvn"com.lihaoyi::scalatags:${millbuild.DepVersions.scalatagsVersion}",

example/extending/metabuild/5-meta-shared-sources/build.mill

@@ -2,7 +2,7 @@ package build
 
 import mill._, scalalib._
 
-object `package` extends RootModule with ScalaModule {
+object `package` extends ScalaModule {
   def scalaVersion = millbuild.ScalaVersion.myScalaVersion
   // Add (or replace) source folders for the module to use
   def customSources = Task.Sources("mill-build/src")

example/extending/plugins/7-writing-mill-plugins/myplugin/test/resources/example-test-project/build.mill

@@ -4,7 +4,7 @@ package build
 
 import mill._, myplugin._
 
-object `package` extends RootModule with LineCountJavaModule {
+object `package` extends LineCountJavaModule {
   def lineCountResourceFileName = "line-count.txt"
 }
 

example/extending/plugins/7-writing-mill-plugins/myplugin/test/resources/integration-test-project/build.mill

@@ -4,6 +4,6 @@ package build
 
 import mill._, myplugin._
 
-object `package` extends RootModule with LineCountJavaModule {
+object `package` extends LineCountJavaModule {
   def lineCountResourceFileName = "line-count.txt"
 }

example/fundamentals/modules/7-root-module/build.mill

@@ -4,7 +4,7 @@
 package build
 import mill._, javalib._
 
-object `package` extends RootModule with JavaModule {
+object `package` extends JavaModule {
   def mvnDeps = Seq(
     mvn"net.sourceforge.argparse4j:argparse4j:0.9.0",
     mvn"org.thymeleaf:thymeleaf:3.1.1.RELEASE"

example/javalib/dependencies/1-mvn-deps/build.mill

@@ -2,7 +2,7 @@
 package build
 import mill._, javalib._
 
-object `package` extends RootModule with JavaModule {
+object `package` extends JavaModule {
   def mvnDeps = Seq(
     mvn"com.fasterxml.jackson.core:jackson-databind:2.13.4"
   )

example/javalib/dependencies/3-unmanaged-jars/build.mill

@@ -2,7 +2,7 @@
 package build
 import mill._, javalib._
 
-object `package` extends RootModule with JavaModule {
+object `package` extends JavaModule {
   def unmanagedClasspath = Task {
     if (!os.exists(moduleDir / "lib")) Seq()
     else Seq.from(os.list(moduleDir / "lib").map(PathRef(_)))

example/javalib/dependencies/4-downloading-unmanaged-jars/build.mill

@@ -2,7 +2,7 @@
 package build
 import mill._, javalib._
 
-object `package` extends RootModule with JavaModule {
+object `package` extends JavaModule {
   def unmanagedClasspath = Task {
     if (Task.offline) Task.fail("Cannot download classpath when in offline-mode") // <1>
     else {

example/javalib/linting/1-error-prone/build.mill

@@ -8,7 +8,7 @@ package build
 
 import mill._, javalib._, errorprone._
 
-object `package` extends RootModule with JavaModule with ErrorProneModule {
+object `package` extends JavaModule with ErrorProneModule {
   def errorProneOptions = Seq("-XepAllErrorsAsWarnings")
 }
 

example/javalib/linting/2-checkstyle/build.mill

@@ -4,7 +4,7 @@
 package build
 import mill._, javalib._, checkstyle._
 
-object `package` extends RootModule with CheckstyleModule {
+object `package` extends CheckstyleModule {
   def checkstyleVersion = "9.3"
 }