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 = ...; }. Supported
plugin formats are zip-files and directories.

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. I'm guessing it's an upstream issue.