This fixes the bug introduced by 8686b98612 which broke bundlerEnv
exprs when gemdir was a string (thus making gemset a string by
`gemset = gemdir + "/gemset.nix"`) which made it being treated as a
set.
Keep the `source` attrset distinct to prevent its entries from merging
with the top level attrs.
Since each type of source has a different set of entries for `source`,
this is the easiest way to keep them together.
This will pave the way for a new `url` type of source.
This is a mass-rebuild of many ruby packages,
notably those using `git` type sources.
This allows patching the gemset output by bundix from a default.nix
file, making it easier to perform updates since the bundix update no
longer has to be manually updated.
