Handle parts of paths

[nrc]

In a path a::b::c::d we handle the a and the d, but not b or c. We used to have a hack in DXR to do some approximation of name resolution, but that is dumb. We should change the compiler to give spans and name resolution info for each path segment, especially as that info would be useful elsewhere in the compiler. Unfortunately, I think the required changes are quite complex.