fix workspace discovery & sorting when folder has an extension

[syphar]

When trying to add cargo sort to docs.rs, I stumbled onto an odd issue:

cargo sort --workspace just gave me an error:

error: no file found at: /Users/syphar/src/rust-lang/docs.rs/

Looking through the source, the culprit seems to be that we're checking for an extension instead of checking if it's a directory before adding a Cargo.toml suffix / filename?

Manual testing is good after this change here.

Are there unittest for this I just didn't see?

Should we add some?