Derived new GenIdea module to support meta-builds (#2638)

Moved the implementation from `mill.scalalib` to a new module
`mill.idea`. The reason is simple, we need access to the `runner`
module, but `runner` itself depends on `scalalib` to provide compilation
support. Generating an IDE setup has a much broader scope so a dedicated
module is IMHO the best. This is analog to `mill.bsp`, which also
provides IDE support as a dedicated module.

Deprecated the old `mill.scalalib.GenIdea` module.

Enhanced the lookup for modules and root modules to reflect the new
meta-build capabilities of Mill.

Reworked the XML generator to re-respect the
`GenIdeaModule.intellijModulePath` setting and ensured we combine
multiple source folders under the same source root if appropriate.

Pull request: #2638

build.sc

@@ -1247,13 +1247,17 @@ object runner extends MillPublishScalaModule {
   }
 }
 
+object idea extends MillPublishScalaModule {
+  def moduleDeps = Seq(scalalib, runner)
+}
+
 object dist extends MillPublishJavaModule {
   def jar = dev.assembly()
-  def moduleDeps = Seq(runner)
+  def moduleDeps = Seq(runner, idea)
 }
 
 object dev extends MillPublishScalaModule {
-  def moduleDeps = Seq(runner)
+  def moduleDeps = Seq(runner, idea)
 
   def testTransitiveDeps = super.testTransitiveDeps() ++ Seq(
     runner.linenumbers.testDep(),
@@ -1437,7 +1441,7 @@ object docs extends Module {
     PathRef(workDir / "build" / "site")
   }
 
-  def source0: Source = T.source(millSourcePath)
+  def source0 = T.source(millSourcePath)
   def source = T {
     os.copy(source0().path, T.dest, mergeFolders = true)
 

docs/modules/ROOT/pages/Installation_IDE_Support.adoc

@@ -291,7 +291,7 @@ To generate IntelliJ IDEA project files into `.idea/`, run:
 
 [source,bash]
 ----
-mill mill.scalalib.GenIdea/idea
+mill mill.idea.GenIdea/idea
 ----
 
 == Updating Mill

idea/src/mill/idea/GenIdea.scala

@@ -0,0 +1,25 @@
+package mill.idea
+
+import mill.T
+import mill.api.Result
+import mill.define.{Command, Discover, ExternalModule}
+import mill.eval.Evaluator
+
+import scala.util.control.NonFatal
+
+object GenIdea extends ExternalModule {
+
+  def idea(allBootstrapEvaluators: Evaluator.AllBootstrapEvaluators): Command[Unit] = T.command {
+    try {
+      Result.Success(GenIdeaImpl(
+        evaluators = Evaluator.allBootstrapEvaluators.value.value
+      ).run())
+    } catch {
+      case GenIdeaImpl.GenIdeaException(m) => Result.Failure(m)
+      case NonFatal(e) =>
+        Result.Exception(e, new Result.OuterStack(new java.lang.Exception().getStackTrace))
+    }
+  }
+
+  override lazy val millDiscover = Discover[this.type]
+}