Display all exports and re-exports

[euclio]

If an item isn't reachable through it's qualified path or is re-exported at another location, rustdoc doesn't display it.

This is currently blocked on

• Provide an API to get the visibility of an item rust-dev-tools/rls-analysis#79
• Provide an API to get imports, particularly pub use rust-dev-tools/rls-analysis#123

I'm currently working through the rls and compiler changes necessary for this, but I'll be tracking the progress here.