feat: allow scip-java index --output to work for Mill

This ensures that when a user uses `scip-java` with the output flag
like:
```
scip-java index --output dump.lsif
```
Then this gets forwarded to Mill to ensure it creates an lsif file not a
scip one.

refs: #482

Author: ckipp01

scip-java/src/main/scala/com/sourcegraph/scip_java/buildtools/MillBuildTool.scala

@@ -35,6 +35,8 @@ class MillBuildTool(index: IndexCommand) extends BuildTool("mill", index) {
     1
   }
 
+  private val rawOutput = index.output.toString
+
   private def unconditionallyGenerateScip(): Int = {
     val localMill = Files.isRegularFile(millFile)
     val command =
@@ -48,7 +50,9 @@ class MillBuildTool(index: IndexCommand) extends BuildTool("mill", index) {
         command,
         "--import",
         s"ivy:io.chris-kipp::mill-scip::${BuildInfo.millScipVersion}",
-        "io.kipp.mill.scip.Scip/generate"
+        "io.kipp.mill.scip.Scip/generate",
+        "--output",
+        rawOutput
       )
     )
     val scipFile = index
@@ -60,10 +64,10 @@ class MillBuildTool(index: IndexCommand) extends BuildTool("mill", index) {
       .resolve("scip")
       .resolve("Scip")
       .resolve("generate.dest")
-      .resolve("index.scip")
+      .resolve(rawOutput)
 
     if (millProcess.exitCode == 0 && Files.isRegularFile(scipFile)) {
-      val output = index.workingDirectory.resolve("index.scip")
+      val output = index.workingDirectory.resolve(rawOutput)
       Files.copy(scipFile, output, StandardCopyOption.REPLACE_EXISTING)
       index.app.info(output.toString)
     }

tests/buildTools/src/test/scala/tests/MillBuildToolSuite.scala

@@ -118,4 +118,23 @@ class MillBuildToolSuite extends BaseBuildToolSuite {
     initCommand = setupMill("0.10.7"),
     targetRoot = Some("out/io/kipp/mill/scip/Scip/generate.dest")
   )
+
+  checkBuild(
+    "lsif-output",
+    s"""|/.mill-version
+        |0.10.7
+        |/build.sc
+        |import mill._, scalalib._
+        |object minimal extends ScalaModule {
+        |  def scalaVersion = "3.1.3"
+        |}
+        |/minimal/src/Main.scala
+        |package minimal
+        |@main def hello = ()
+        |""".stripMargin,
+    expectedSemanticdbFiles = 1,
+    initCommand = setupMill("0.10.7"),
+    targetRoot = Some("out/io/kipp/mill/scip/Scip/generate.dest"),
+    extraArguments = List("--output", "dump.lsif")
+  )
 }