Find function path references early in the same lint pass

[y21]

This removes a visitor that existed to collect paths to functions in a context where the exact signature is required in order to cancel the lint.
E.g. when there's a let _: fn(&mut i32) = path_to_fn_ref_mut_i32; statement somewhere in the crate, we shouldn't suggest removing the mutable reference in the function signature.

It was doing a whole pass through the crate at the end, which seems unnecessary.
It seems like we should be able to add entries to the map in the same lint pass.
The map is untouched all the way until check_crate_post (at which point it will be populated by the visitor and finally checked), so it doesn't seem like this changes behavior: it will only be fully populated by the time we reach check_crate_post no matter what.

I don't think this will have a significant perf impact but it did show up in a profile with 0.5% for a crate I was looking into and looked like a low hanging fruit.

changelog: none

[rustbot]

r? @llogiq

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[llogiq]

Thank you!

@bors r+

[bors]

📌 Commit 0774489 has been approved by llogiq

It is now in the queue for this repository.

[bors]

⌛ Testing commit 0774489 with merge 70573af...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: llogiq
Pushing 70573af to master...