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Rename package deviceplugin => devicemanager.
**What this PR does / why we need it**:
Fixes#58795
/kind cleanup
Rename package `deviceplugin` to `devicemanager` for consistency.
We already have components named Container manager and CPU manager. The device plugin package similarly contains an interface called `Manager`. The fact that the manager has plugins is somewhat incidental to the purpose of the package itself.
Note that this rename only affects internal API. The external gRPC interface still exports a package called deviceplugin.
**Release note**:
```release-note
NONE
```
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