checker: fix module documentation and style
checker: restrict handling of plugin files to the registry

* Remove `plugin.py` as it was no longer in use. Any future related
  additions should be done in `registry.py`.

* Move PluginMissing exception to `errors.py`.
checker: intbitset for resolving user requrested IDs

* Use intbitset's integer sanity checks.

* Discontinue use of argparse:ArgumentTypeError to drop direct
  dependency on argparse and cleanly separate concerns. #r20922678
checker: remove resolution of `Rules` from type-checking
Rename internal refernces of `ids` to `recids`
checker: utilize workflows helper functions for checks

* Use `obj.extra_data` instead of `obj.data` for initial data.

* Leverage information from configuration and modules to extract output
  from pre-functions and construct argument lists.