rustpkg makes it difficult to work with local changes to submodules

[catamorphism]

Currently, there is no easy way when using rustpkg to write:

extern mod foo = "github.com/mozilla/whatever";

and concurrently be developing the whatever package, with local patches that aren't in the github repo. You can make local changes, but rustpkg will overwrite them (although you shouldn't be able to, as per #6480). If you instead check out the sources for whatever into a different directory and work on the copy in that directory, you have to change the extern mod directive and, for the benefit of anyone else using your package, bundle the sources for whatever as a submodule. This is not ideal. I'm not sure how to reconcile wanting to name packages by their URLs with wanting to make local changes to them.

/cc @metajack since we talked about this before.

[catamorphism]

I think this needs to be fixed in concert with #6480 , otherwise there's too much potential for confusion in the meantime.

[catamorphism]

This can close when #9732 lands. See the tests::test_installed_local_changes for how it's supposed to work.

[catamorphism]

#9732 fixed this.