Prior to this commit we used to generate import paths, then zipped them with the existing qualifier to check if they agree on the path to import.
This is brittle when re-exports come into play causing items to have multiple applicable paths that refer to them.
This commit instead rewrites this logic by generating the import path for the qualifier, verifying that the rest of the qualifier resolves and then doing a final lookup on that resolution result for the final segment instead.
This PR touches a lot of parts. But the main changes are changing
`hir_expand::Name` to be raw edition-dependently and only when necessary
(unrelated to how the user originally wrote the identifier),
and changing `is_keyword()` and `is_raw_identifier()` to be edition-aware
(this was done in #17896, but the FIXMEs were fixed here).
It is possible that I missed some cases, but most IDE parts should properly
escape (or not escape) identifiers now.
The rules of thumb are:
- If we show the identifier to the user, its rawness should be determined
by the edition of the edited crate. This is nice for IDE features,
but really important for changes we insert to the source code.
- For tests, I chose `Edition::CURRENT` (so we only have to (maybe) update
tests when an edition becomes stable, to avoid churn).
- For debugging tools (helper methods and logs), I used `Edition::LATEST`.
