Split def_path_res into two parts

[GnomedDev]

def_path_res previously had two jobs:

1. looking up the crates to find the path in
2. looking up path in said crates

This splits that job up into two functions, keeping def_path_res as an adapter between the both, to avoid repeating the first step when repeatedly looking up items in the same crate.

changelog: none

[rustbot]

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[bors]

☔ The latest upstream changes (presumably #13440) made this pull request unmergeable. Please resolve the merge conflicts.

[y21]

Thanks!

@bors r+

[bors]

📌 Commit 6b34c8d has been approved by y21

It is now in the queue for this repository.

[bors]

⌛ Testing commit 6b34c8d with merge 8ab744e...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: y21
Pushing 8ab744e to master...