Allow package IDs to refer to the middle of a source repository

[catamorphism]

Sub-bug of #5677

Currently, rustpkg assumes that a remote package ID refers to the root level of a particular source repository. @brson suggested that to obviate the need for servo to use git submodules, we could allow package IDs to point into a subdirectory of a repo. It's unclear whether we should implement this. See discussion at https://etherpad.mozilla.org/Rustpkg-Servo-meeting

[emberian]

This seems like a good idea to me. Imagine a C library that ships rust bindings in a 'rust' subdirectory.

[lilyball]

Go allows you to reference a sub-path of a repo. It checks out the correct repo, and only builds that sub-path.

[lilyball]

If this is implemented (and I think it should be), then rustpkg should also stop recursively searching for lib.rs (and the other magic files) and instead require them to be in the root. This is because rustpkg assumes that the last path component of the pkgid is the name of the library, and it would be very confusing if I could type both rustpkg install github.com/foo/bar and rustpkg install github.com/foo/bar/baz and have both build the same library, but only the latter successfully install.