scala version detection: check dependencies before classpath

Now, the version is primarily detected from project dependencies before falling back to the classpath. This is due to the classpath possibly containing more items than direct library dependencies, which could lead to inaccurate version detection.

Related:
- #126
- #111

core/src/main/scala/org/jetbrains/sbtidea/structure/sbtImpl/package.scala

@@ -45,13 +45,16 @@ package object sbtImpl {
   }
 
   def detectScalaVersion(projectData: CommonSbtProjectData): ProjectScalaVersionImpl = {
+    val mainScalaLibraryFormDeps = detectMainScalaLibrary(projectData.definedDeps)
     val mainScalaLibraryFromClasspath = detectMainScalaLibrary(projectData.cp.flatMap(_.metadata.get(sbt.Keys.moduleID.key)))
-
-    val mainScalaLibrary = if (mainScalaLibraryFromClasspath.nonEmpty)
-      mainScalaLibraryFromClasspath
-    else
-      detectMainScalaLibrary(projectData.definedDeps)
-
+    // First detect scala version form dependencies and only then form the classpath. Classpath entire can include more
+    // items then direct libraries dependencies. If a Scala 2 module depends on a Scala 3 module, we need to consider
+    // the module Scala 2 anyway. But the classpath contain Scala 3 even though direct dependencies won't.
+    // Thus we first check dependencies only.
+    //
+    // NOTE: it seems like the most straight-forward way would be not "guessing" but propagating the version form the project "scalaVersion" key
+    // We could reimplement it in future
+    val mainScalaLibrary = mainScalaLibraryFormDeps.orElse(mainScalaLibraryFromClasspath)
     ProjectScalaVersionImpl(mainScalaLibrary)
   }
 }