Ability to target specific message files in a package.

[Carter12s]

Ran into a use case where some folks had a massive single package full of all of their messages.

A crate may not want to generate ALL the messages in that package, but only a subset. For now our codegen only operates by finding packages, but I think we could allow a way to only specify certain messages as "targets". We would still need to walk dep tree to find what those messages reference, but it could be very nice to allow this.