Use correct span in metadata::creader::resolve_crate_deps

[catamorphism]

There's a FIXME in creader::resolve_crate_deps about using an accurate span instead of a bogus span.

[bstrie]

Triage here. The aforementioned FIXME persists.

[emberian]

Still relevant. I think the FIXME is for when resolving the deps of an extern mod, the span should point to that extern mod when it fails, rather than nowhere.

[emberian]

To fix this, resolve_crate_deps needs to be passed the span of the crate it's resolving, I think.

[pzol]

Bump for triage

[cmacknz]

I'll take this, looks fairly straight forward.