Enable Source Lookup for PDE projects

• Import org.jboss.tools.maven.sourcelookup.core
• open a class from maven-core-3.0.4.jar in the Plugin dependencies
• source lookup should kick in but it doesn't

PDE actually assign the sourcePath to maven-core-3.0.4.jar itself.
Source lookup only checks a sourcePath file is set and exists. It should also check the attached source actually contains a matching source file.

I have some stuff ready locally. Will publish tomorrow.