[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-scala/pom.xml

@@ -0,0 +1,25 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <artifactId>pmd-scala</artifactId>
+    <name>PMD Scala - Transitional package (deprecated)</name>
+    <description>This is deprecated, use pmd-scala_2.13 directly</description>
+    <packaging>pom</packaging>
+
+    <parent>
+        <groupId>net.sourceforge.pmd</groupId>
+        <artifactId>pmd</artifactId>
+        <version>6.25.0-SNAPSHOT</version>
+        <relativePath>../</relativePath>
+    </parent>
+
+    <dependencies>
+        <dependency>
+            <groupId>${project.groupId}</groupId>
+            <artifactId>pmd-scala_2.13</artifactId>
+            <version>${project.version}</version>
+        </dependency>
+    </dependencies>
+</project>