rustc: Fix `extern crate` being order dependent

rustc: Fix `extern crate` being order dependent

This commit fixes a bug where a crate could fail to compile depending on the
order of `extern crate` directives at the top of the crate. Specifically, if the
same crate is found at two locations, then if it's loaded first via `--extern`
it will not emit a duplicate warning, but if it's first loaded transitively
via a dep and *then* via `--extern` an error will be emitted.

The loader was tweaked to catch this scenario and coalesce the loading of these
two crates to prevent errors from being emitted.