Meta-packages

This allows a single git repository to have multiple packages contained inside.

* Cargo will look for `Cargo.toml`s located anywhere inside the project.
* Cargo will stop searching once it reaches a submodule, to avoid accidentally exporting someone else's packages. It is still possible to explicitly depend on packages from a submodule using the `path=` syntax on dependencies.
Ah, so the important part of the test is the `meta-dep` having two `Cargo.toml` files, and `parent` is just checking that importing them (etc.) all works?