Suppose I have a Gemfile like this:

    source "https://rubygems.org"
    gem "actioncable"
    gem "websocket-driver", group: :test

The gemset.nix generated by Bundix 2.4.1 will set ActionCable's groups
to [ "default" ], and websocket-driver's to [ "test" ]. This means that
the generated bundlerEnv wouldn't include websocket-driver unless the
test group was included, even though it's required by the default group.

This is arguably a bug in Bundix (websocket-driver's groups should
probably be [ "default" "test" ] or just [ "default" ]), but there's no
reason bundlerEnv should omit dependencies even given such an input --
it won't necessarily come from Bundix, and it would be good for
bundlerEnv to do the right thing.

To fix this, filterGemset is now a recursive function, that adds
dependencies of gems in the group to the filtered gemset until it
stabilises on the gems that match the required groups, and all of their
recursive dependencies.

This wasn't really an issue until the latest minor release of Bundix
(2.4), because prior to then Bundix didn't emit group attributes, and so
this functionality of bundlerEnv wasn't really used. However, it is now
apparent that a better default for bundlerEnv would be to include all
gem groups by default, not just the default group. This matches the
behavior of Bundler, and makes more sense, because the default group
alone isn't necessarily useful for anything -- consider a Rails app with
production, development, and test groups. It has the additional benefit
of being backwards compatible with how this would have worked before the
Bundix update.

"default" isn't really a group, it's more the absence of one. With
Bundler, this means that a gem should be installed unconditionally,
regardless of which groups are specified. It doesn't really make sense
to allow these gems to be omitted from a bundlerEnv.

bundlerEnv: improve handling of groups