rustpkg: Add support for custom configuration for dependencies

[z0w0]

Rustpkg allows you to do rustpkg install --cfg image git://github.com/brson/rust-sdl.git but it doesn't let you do #[pkg_dep(url = "git://github.com/brson/rust-sdl.git", cfg = "image")];. NB. Rustpkg hasn't landed yet, but it has been approved.

[catamorphism]

I'm not actually sure what we're going to do about dependencies on remote packages; I know for local dependencies, we'll infer them from extern mod directives.

[catamorphism]

This is obsolete; see #6407