cpanfile: add all xref dependencies, via a feature

To begin with, uncomment both features. I do not think I even want
to know what the logic behind having this commented out in the first
place was. It must have happened before Ensembl got migrated to GitHub
though, judging from commit history.

Secondly, update the list of dependencies of the xref_mapping feature
so that it reflects what is really needed (read: mostly add missing
ones).

Finally, get rid of misc-scripts/xref_mapping/cpanfile .