Speculative crate loading is not speculative enough

[petrochenkov]

Speculative crate loading is performed as a part of speculative name resolutions (fn maybe_process_path_extern) which is used for various import suggestions, lints and similar things.

It's supposed to be silent and return None if the crate cannot be loaded, however it still reports an error if the crate path doesn't exist.
This shouldn't happen and crate path errors should be delayed until we know whether the crate loading is speculative or not.

After this issue is fixed, the test change in #55102 needs to be reverted.

cc @davidtwco @eddyb

[petrochenkov]

Fixed in #74071.