[scala] Refactor project structure

* Avoid using symlinks, this is very difficult to get working
  under Windows
* Provide names for the different scala modules
* Rename pmd-scala to pmd-scala-common
* Restore main module pmd-scala for backwards compatibility

pmd-lang-test/src/main/kotlin/net/sourceforge/pmd/lang/ast/test/BaseTreeDumpTest.kt

@@ -51,7 +51,7 @@ abstract class BaseTreeDumpTest(
 
     // Outputting a path makes for better error messages
     private val srcTestResources = let {
-        // this is set from maven surefire
+        // this is set from maven surefire - see parent pom.xml configuration for surefire (systemPropertyVariables)
         System.getProperty("mvn.project.src.test.resources")
                 ?.let { Paths.get(it).toAbsolutePath() }
                 // that's for when the tests are run inside the IDE

pmd-scala-modules/pmd-scala-common/README.md

@@ -0,0 +1,8 @@
+# pmd-scala-common
+
+Contains shared code for the pmd-scala_2.12 and pmd-scala_2.13 modules.
+
+It is not a module in itself, since it needs to be used while cross-compiling
+with a specific scala version.
+
+It should be source compatible with the oldest scala version we are supporting.