Some plugin repos already have a `gems` directory. This lets the
packager choose whether it should be kept and the nix packaged ruby
gems should be copied into it or if it should be removed in favor of
our ruby gems.
Perform the tests on the package that the `tests` attribute is a child
of, i.e. if `discourseAllPlugins.tests` is built, the tests will run
with the `discourseAllPlugins` package, not the `discourse` package as
previously.
Some discourse plugins have Ruby dependencies and require a
specialized builder. This introduces a generic builder that can be
used whether the plugin has Ruby dependencies or not. It also adds a
set of pre-packaged plugins available through `discourse.plugins` and
provides an easy way to add more.
For plugins to work properly, their assets need to be precompiled
along with the rest of Discourse's assets. This means we need to build
new packages when the list of plugins change.
