Provide better diagnostics when patching fails

[jdm]

There is one particular case I keep hitting that's a papercut for me. Servo uses a mixture of git and crates.io dependencies, and sometimes we end up switching between the two so it's easy to forget the true source of a dependency. When adding a patch, we either need a [patch.crates-io] or [patch."https://github.com/servo/whatever"]. When I get this wrong, however, all I get is a message from Cargo that says that the patch was ignored. It seems like it should be possible for Cargo to notice when a patch matches the name of an existing dependency, check whether the patch source differs from the dependency's source and provide a better error message in that case.

[benesch]

I'm with you 100%—I've been real frustrated by this! But this is arguably a duplicate of #6341?

[jdm]

It overlaps, but I think keeping them separate makes sense - this one is particularly focused on the poor diagnostics.