The way ruby loads gems and keeps track of their paths seems to not
always work very well when the gems are accessed through
symlinks. Ruby will then complain that the same files are loaded
multiple times; it relies on the file's full path to determine whether
the file is loaded or not.
This adds an option to simply copy all gem files into the environment
instead, which gets rid of this issue, but may instead result in major
file duplication.
The bin stubs need to be built where there's access to /nix/store - so
it can't happen in a nix-shell run. Ergo, a shell.nix needs to be able
to signal to the build that all bins need to be built.
`usesGemspec` no longer required to trigger the "copy everything into
gemfile-and-lock" behavior. If the mainGem is referred to by path,
that's sufficient.
Add `pname` attribute: if passed, the derivation name defaults to the
gem name + version and only expose the gem's bin.
Add `gemdir` attribute: gives a default lookup path for the Gemfile,
Gemfile.lock and gemset.nix.
Set the `meta.platforms' to `ruby.meta.platforms' by default.
