Tracking issue for RFC #532: change `mod` to `self` in use items

[nrc]

[nrc]

RFC

[nrc]

Step 1: accept both syntaxes, the old one is deprecated (#20365)
Step 2: snapshot
Step 3: convert uses of old syntax (https://github.com/nick29581/rust/tree/mod2)
Step 4: remove old syntax

[nrc]

This has been fixed