Add support for non-crate dependencies

[gyakovlev]

Sometimes a project uses a git dependency.
cargo-ebuild should handle simple cases of git deps.

[leonardohn]

The main changes required for this to work should be on cargo.eclass. There should be a variable similar to CRATES to allow the maintainer to add vendored dependency paths relative to the workdir. It could be done by generating (or modifying) the .cargo/config file so cargo can handle those dependencies.

[thesamesam]

I suppose this covers the case I mentioned in gentoo/pycargoebuild#14 (comment) wrt pkgcraft-c's '../pkgcraft' dep too?

[gyakovlev]

Not really, that’s different. It’s already working fine if you invoke cargo(eclass included) properly.