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.
Fix dropled directory naming
Don't install POM files for Eclipse artifacts
Avoid artifact metadata duplication
Fail if artifact path cannot be determined

It seems to be better than returning "/dev/null".
Fix platform/internal/external unit querying logic
Improve debugging messages in DefaultEclipseInstaller
Don't assume that main package has ID equal to empty string

Fixes: #2
Fix metapackage merging
Prevent packages from adding self-dependencies
Allow installing the same symlink into separate dropins