[MRESOLVER-515] Restore binary compatibility (apache#446)

In cases when AbstractArtifact was extended, the
getPath method would go missing.

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https://issues.apache.org/jira/browse/MRESOLVER-515

Author: cstamas

maven-resolver-api/src/main/java/org/eclipse/aether/artifact/AbstractArtifact.java

@@ -96,6 +96,15 @@ public Artifact setVersion(String version) {
         return newInstance(version, getProperties(), getPath());
     }
 
+    /**
+     * This method should (and in Resolver is) overridden, but is kept just to preserve backward compatibility if
+     * this class is extended somewhere.
+     */
+    public Path getPath() {
+        File file = getFile();
+        return file != null ? file.toPath() : null;
+    }
+
     @Deprecated
     @Override
     public Artifact setFile(File file) {