Disambiguate exclude patterns to avoid upcoming warnings

[mjambon]

1. Avoid upcoming warnings about unanchored patterns that will become anchored. These are paths.include and paths.exclude glob patterns containing a middle slash but not starting with ** or /.
2. Ensure we preserve the current behavior when the implementation changes at the end of the deprecation period.

I already submitted a similar PR a few days ago (#3630) but it only took care of the includes. Since then, we've decided to emit a warning for all include and exclude patterns that will become anchored after a deprecation. This PR addresses excludes.