Allow for targeted git dependencies

[wagenet]

See #3195

[wagenet]

@alexcrichton can you verify that these tests should pass?

[alexcrichton]

I don't think that duplicate_deps_2 can probably pass, unfortunately, (but it'd be nice if it did!). The latter one would be nice to pass, yeah!

[wagenet]

@alexcrichton the issue here is that the check for conflicts check for conflicting source_id (https://github.com/rust-lang/cargo/blob/master/src/cargo/util/toml.rs#L628). The dubplicate_deps_2 passes because all crates specified by version always have the same source_id, despite being different deps. When specified by path or git the source_ids vary and it's deemed a conflict. What do you think the correct approach to resolving this is?

[alexcrichton]

I've always felt pretty uneasy about multiple entries in the list of dependencies with the same name, if those are let through I suspect it will wreak havoc on the backend which wasn't expecting that at all. In that sense this likely won't be an easy bug to fix. Unfortunately I don't know the best way to tackle this right now :(

[wagenet]

I have an actual case for this where I'm trying to support both old libc as well as FreeBSD. The versions of libc required for the two cases are incompatible. Because I can't switch between two version of the same library, I've actually had to resort to cloning the respective crates and giving them a different name.

[bors]

☔ The latest upstream changes (presumably #4259) made this pull request unmergeable. Please resolve the merge conflicts.

[alexcrichton]

I'm going to close this for now due to activity, but please feel free to resubmit of course!

[wagenet]

See #6212