Maven Sync: Don't create absolute symbolic links if possible

[haubi]

The "Synchronize Maven Artifacts" feature creates symbolic links from the bundle directory to the Maven cache directory.
Absolute symbolic links are in general a source of potential problems and should be avoided, if possible

Suggestion: if the cache directory and the bundle directory are both children of the workspace directory, relative symbolic links should be generated.

Initially reported at https://bugs.eclipse.org/bugs/show_bug.cgi?id=540080