Allow resolving moduleDeps with older Scala 3 versions (#2877)

# Motivation
Sometimes you want different modules in a build to have different Scala
3 versions, one the older and one the newer.
The current resolve implementation in `CrossModuleBase` is tailored to
Scala 2 since it assumes that the binary version is `x.y` while in Scala
3 it's just the first part.

This change adapts the algorithm to consider 1 or 2 parts depending if
it is Scala 3 or not.

Pull Request: #2877

scalalib/src/mill/scalalib/CrossModuleBase.scala

@@ -22,7 +22,7 @@ trait CrossModuleBase extends ScalaModule with Cross.Module[String] {
         crossScalaVersion
           .split('.')
           .inits
-          .takeWhile(_.length > 1)
+          .takeWhile(_.length > (if (ZincWorkerUtil.isScala3(crossScalaVersion)) 0 else 1))
           .flatMap(prefix =>
             c.crossModules
               .find(_.crossScalaVersion.split('.').startsWith(prefix))

scalalib/test/src/mill/scalalib/HelloWorldTests.scala

@@ -22,6 +22,8 @@ object HelloWorldTests extends TestSuite {
   val scala2123Version = "2.12.3"
   val scala212Version = sys.props.getOrElse("TEST_SCALA_2_12_VERSION", ???)
   val scala213Version = sys.props.getOrElse("TEST_SCALA_2_13_VERSION", ???)
+  val scala32Version = sys.props.getOrElse("TEST_SCALA_3_2_VERSION", ???)
+  val scala33Version = sys.props.getOrElse("TEST_SCALA_3_3_VERSION", ???)
   val zincVersion = sys.props.getOrElse("TEST_ZINC_VERSION", ???)
 
   trait HelloBase extends TestUtil.BaseModule {
@@ -59,6 +61,15 @@ object HelloWorldTests extends TestSuite {
         )
     trait HelloWorldCross extends CrossScalaModule
   }
+  object CrossModuleDeps extends HelloBase {
+    object stable extends Cross[Stable](scala212Version, scala32Version)
+    trait Stable extends CrossScalaModule
+
+    object cuttingEdge extends Cross[CuttingEdge](scala213Version, scala33Version)
+    trait CuttingEdge extends CrossScalaModule {
+      def moduleDeps = Seq(stable())
+    }
+  }
 
   object HelloWorldDefaultMain extends HelloBase {
     object core extends HelloWorldModule
@@ -677,6 +688,16 @@ object HelloWorldTests extends TestSuite {
       }
     }
 
+    "scala-33-depend-on-scala-32-works" - {
+      CrossModuleDeps.cuttingEdge(scala33Version).moduleDeps
+    }
+    "scala-213-depend-on-scala-212-fails" - {
+      val message = intercept[Exception](
+        CrossModuleDeps.cuttingEdge(scala213Version).moduleDeps
+      ).getMessage
+      assert(message == "Unable to find compatible cross version between 2.13.8 and 2.12.6,3.2.0")
+    }
+
     "runMain" - {
       "runMainObject" - workspaceTest(HelloWorld) { eval =>
         val runResult = eval.outPath / "core" / "runMain.dest" / "hello-mill"