Avoid extracting tycho-bundles-external.zip in XMvn plugin

XMvn P2 plugin used OpenJDK, which doesn't support extracting ZIP
files with embedded symlinks. This would require Plexus Archiver. But
even then extracting tycho-bundles-external.zip would be inefficient
as all linked bundles are already available in the system.

Instead this commit makes XMvn P2 obtain absolute paths to external
bundles by examining contents of tycho-bundles-external-manifest.txt
file, will which contain list of paths to external bundles and which
will be generated during build.