Add support for combining conflicting exported names.

[jonmeow]

I think this is a key remaining bit of polish for intra-package exports. Next I'm going to be dealing with cross-package exports, though I think the fundamentals here will remain intact.

Extern declarations could cause issues for the current approach, but I think that can be addressed separately. I don't think it's a fundamental problem, more just incremental.

[jonmeow]

Depends on #3950 (dependency merged)