eclipse: add support for plugins

Where "support for plugins" means that it is now possible to tell
Eclipse about extra plugins in the nix expression. The specified plugins
will simply be copied to the Eclipse "dropins/" directory.

The buildEclipse function is made overridable so that extra plugins can
be specified as some-eclipse.override { extraPlugins = ...; }.

With this change it is now possible to have a _deterministic_ deployment
of eclipse + a set of plugins, unlike the standard upstream / Eclipse
way.

Unfortunately, some plugins don't install properly using this scheme; I
tested PyDev and GNU ARM Eclipse and the latter plugin lacked some
components. My guess is that it's an upstream issue, but I haven't
verified it.

According to Eclipse documentation, plugins placed in the dropins/
directory will even have its prerequisites pulled in automatically (if
the aren't satisfied already). I haven't been able to test this in NixOS
though.