Support per-package dependencies/same repo dependencies

[enderger]

Currently, there's no good way to have multiple mutually dependent packages in the same repo. At present, all packages depend on the same list of dependencies, so if you depend on a library in one package it will also be exported depending on itself and any other libraries that it may or may not need.

[mattnite]

This is a downside with how gyro was designed, but I'm focusing my time on other projects so this likely won't get fixed any time soon. I welcome anyone else to add this improvement!