Changes `librustc_X` to `rustc_X`, only in documentation comments.
Plain code comments are left unchanged.

Also fix incorrect file paths.

The constraint set is going to be more than just a set of outlives
constraints.

also promote to its own directory, make local to nll

We are not currently using it for anything; even polonius just uses
the `from_location`.

The overall format is now easier to read. Also, There is now graphviz
output, as well as a `#[rustc_regions]` annotation that dumps internal
state.