rustdoc: correctly clean cross-crate opaque types

[fmease]

Opening a PR for a patch I had lying around for roughly 3 months to motivate myself to finish it.
Fixes bevyengine/bevy#8898 (#113015, subitem (1)).
Part of #113015.

Before/after (cross-crate re-export scenario):

- pub fn rpit_fn() -> impl Fn()
+ pub fn rpit_fn() -> impl Fn() -> bool
- pub fn rpit_fn_mut() -> impl for<'a> FnMut(&'a str)
+ pub fn rpit_fn_mut() -> impl for<'a> FnMut(&'a str) -> &'a str

Draft: See FIXMEs.

r? @ghost

[bors]

☔ The latest upstream changes (presumably #117405) made this pull request unmergeable. Please resolve the merge conflicts.

[Dylan-DPC]

@fmease any updates on it?

r? rustdoc

[fmease]

Ye, this PR could be merged as is (once I've removed some of the temporary FIXME comments). It's a draft since it's not super DRY and it should be generalized to fix more issues (some of which are mentioned in #113015). However, that'd require rewriting a lot of things which I didn't have the energy to do yet. Imma decide how to proceed today or in the coming days.

[bors]

☔ The latest upstream changes (presumably #121370) made this pull request unmergeable. Please resolve the merge conflicts.

[Dylan-DPC]

@fmease any updates on this? thanks

[fmease]

Coincidentally I will send a new and more extensive PR in a few hours hopefully 🤞, one that should fix #113015 completely. Closing this PR.