Auto merge of #3029 - matklad:workspace-overrides, r=alexcrichton

Don't special case root package for overrides

Hi! What is the expected behavior of path overrides and workspaces? There are some [tests] for this, but looks like they don't actually test the behavior: this commit removes special casing of the root package, but `override_self` still passes. I see two options:

1. Allow overriding of the workspace members as usual. This amounts to removing that `filter`

2. Ignore overriding of the workspace members.

In any case, I think it would be nice to add a test which overrides local package to some other package to make sure that test does not pass simply because the package and its override are in fact the same.

[tests]: https://github.com/rust-lang/cargo/blob/master/tests/path.rs#L599-L679