Only download gems needed for the targets I request

> Note, this is "nice to have", maybe not a requirement for a 1.0 of rules_ruby.

Use case: I'm in a monorepo where one app has 1000 giant dependencies. I write a little ruby tool for all developers to run, and it only needs one small gem, call it "mygem".

I don't want users of my tool to have to wait to download all the giant dependencies. So the `bundler install` would have to be decomposed such that each gem goes in its own external/ repository, and then in my BUILD files instead of depending on "all gems" like `@bundler//:all`, I would depend on `@bundler_mygem`. That way the other gems aren't referenced in the build graph and so they aren't fetched.