Support multiple sources with conflicting crates

[ChrisGreenaway]

It should be possible for cargo-local-registry to have multiple different versions of a crate (i.e. the same crate with the same version from multiple sources).

One way to achieve this would be to have multiple subdirectories in the output registry directory when there are multiple sources (different registries, git repos etc.) - one for each source. The output from running cargo-local-registry would output a .cargo/config file that mapped each of the source registries to the appropriate subdirectory.

I think we should keep the current behaviour (i.e. no subdirectories) when there is just one source. Additionally, we probably want to have a repeatable mapping from the details of a source to the name of the subdirectory, which would have to include, for example, for a git source, the branch, tag or revision. Further, the directory name would need to be valid on all supported filesystems - so I guess at least on UNIX and Windows (others?).

Please see the discussion on #134 for the initial discussion of this idea.

[ChrisGreenaway]

Just noticed that cargo-vendor appears to have something similar with their --no-merge-sources option.