Finish transition from `semitransparent` to `semiopaque` for `rustc_macro_transparency`
Since it's a bit annoying to have different names for the same thing.
My understanding is that this is just internal stuff that is not part of any public API even tough rust-analyzer knows about it.
Continuation of
- https://github.com/rust-lang/rust/pull/139084.
Discovered while investigating
- https://github.com/rust-lang/rust/issues/150514
3daf9935c5