[naga wgsl-in] Try out new use convention.

Change `naga::front::wgsl::lower` to `use` `ir` and `proc`,
and then qualify identifiers with that instead of `crate`.

This is strictly fiddling with `use` directives and name
qualification, with the concomitant formatting changes; there should
be no change to the module's behavior or meaning.

Author: jimblandy