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Move scheduler out of plugin directory

**What this PR does / why we need it**:

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
This is but one step toward resolving the referenced issue.
/ref #57579

**Special notes for your reviewer**:

**Release note**:

```release-note
Default scheduler code is moved out of the plugin directory.
plugin/pkg/scheduler -> pkg/scheduler
plugin/cmd/kube-scheduler -> cmd/kube-scheduler
```
/sig scheduling