Provide better diagnostics when patching fails

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.

Start by tracing Cargo's patch resolution and the diagnostic path that reports an ignored patch. Reproduce the mismatch between a dependency source and a patch source, then verify that the resulting message explains which patch section is required and why the patch was ignored.