Support explicit names in pkgid attributes

This change extends the pkgid attribute to allow of explicit crate names, instead of always inferring them based on the path. This means that if your GitHub repo is called `rust-foo`, you can have your pkgid set your library name to `foo`. You'd do this with a pkgid attribute like `github.com/somewhere/rust-foo#foo:1.0`.

This is half of the fix for #10922.