Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.
Move core API to pkg/apis/core
Fixes https://github.com/kubernetes/kubernetes/issues/55304.
This moves the types in pkg/api and most of pkg/api/v1 to pkg/apis/core to match the other api groups, following-up on what https://github.com/kubernetes/kubernetes/pull/44784 did for the external types.
It's a nearly 100% mechanical package move with a number of trivial fixups in code-generators and in tests which do string comparisons of types via reflection.
**Most importantly:**
- group names are unchanged
- apimachinery semantics are unchanged
- the outside api is unchanged.
Note that some packages for helpers are left in pkg/api/v1. Splitting them into helpers which belong tightly to the api and those which are not is left to follow-ups.
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