Rename InheritanceManager2 to InheritanceHelper/Util/Something and move it out of src

[kevmoo]

1. InheritanceManager2 is not a name we want to keep around. Once InheritanceManager is removed it'll be just...weird...
2. InheritanceManager2 lives under lib/src and is not exported pub a "public" library.

Related to #35833

[kevmoo]

See https://dart-review.googlesource.com/c/sdk/+/92920

[srawlins]

This class is now InheritanceManager3 😛

Can you say why you want this out of lib/src? Sorry for the extreme delay!

[kevmoo]

I use it here: https://github.com/google/json_serializable.dart/blob/190f8b1ea1fdd87943ba230f1deddcc31393a348/json_serializable/lib/src/field_helpers.dart#L8

dartdoc here: https://github.com/dart-lang/dartdoc/blob/2eee9a9231ff29913c693b7e1a0dc32b936b34b7/lib/src/model/library.dart#L14

So it's at thing that's still wanted

[srawlins]

I agree, we use it in mockito as well.

WDYT, @scheglov @bwilkerson? Move InheritanceManager3 to lib/dart/element/, next to, e.g. TypeSystem? Could rename it to InheritanceManager as well, and keep a deprecated reference in the old location.

[scheglov]

Most of the methods that exist in InheritanceManager3 should be just methods of InterfaceElement. So, just moving it and exposing the official API is wrong. We should design something better.

[srawlins]

SG