[Bug] packageExtensions not being applied correctly

[ganemone]

• I'd be willing to implement a fix

Describe the bug

A potential bug with handling package extensions was introduced in this commit. I believe the changes were intended to be refactoring, but not changing behavior of packageExtensions. However, the behavior is now different in certain edge cases. I believe the edge case occurs when a packageExtension is active for one package, but redundant in another.

To Reproduce

See https://github.com/ganemone/yarn-bug/tree/main

[merceyz]

You're running into the final point here #2091 (comment), use resolutions to override existing dependency versions