models: Do not make references for private symbols

[vinzenz]

Previously there was a reference for Models created for every symbol
attempted to being imported from leapp.models, however pytest tries to
import _pytestfixturefunction which is not a model obviously.
To work around pytest causing failures because of that, private symbols
are being filtered from the model reference creation magic.

For reference, this is what you would see running pytest without this fix:

....
INTERNALERROR>     self._inner_hookexec = lambda hook, methods, kwargs: hook.multicall(
INTERNALERROR>   File "/usr/local/lib/python3.9/site-packages/pluggy/callers.py", line 208, in _multicall
INTERNALERROR>     return outcome.get_result()
INTERNALERROR>   File "/usr/local/lib/python3.9/site-packages/pluggy/callers.py", line 80, in get_result
INTERNALERROR>     raise ex[1].with_traceback(ex[2])
INTERNALERROR>   File "/usr/local/lib/python3.9/site-packages/pluggy/callers.py", line 187, in _multicall
INTERNALERROR>     res = hook_impl.function(*args)
INTERNALERROR>   File "/home/vfeenstr/leapp-repository/conftest.py", line 45, in pytest_collectstart
INTERNALERROR>     _load_and_add_repo(collector.session.leapp_repository, current_repo_basedir)
INTERNALERROR>   File "/home/vfeenstr/leapp-repository/conftest.py", line 31, in _load_and_add_repo
INTERNALERROR>     manager.repo_by_id(repo_id).load(skip_actors_discovery=True)
INTERNALERROR>   File "/home/vfeenstr/.local/lib/python3.9/site-packages/leapp/repository/__init__.py", line 135, in load
INTERNALERROR>     resolve_model_references()
INTERNALERROR>   File "/home/vfeenstr/.local/lib/python3.9/site-packages/leapp/models/__init__.py", line 201, in resolve_model_references
INTERNALERROR>     reference.resolve()
INTERNALERROR>   File "/home/vfeenstr/.local/lib/python3.9/site-packages/leapp/models/__init__.py", line 188, in resolve
INTERNALERROR>     raise ModelDefinitionError('Undefined Model "{}"'.format(cls._referenced))
INTERNALERROR> leapp.exceptions.ModelDefinitionError: Undefined Model "_pytestfixturefunction"

[pirat89]

@vinzenz I know we are in time pressure, so in case you want to skip unit-tests now in this PR, please add at least TODO comment and create jira ticket. Regarding we are going to use multiple leapp repositories basically all the time, it will be better to have this field covered by tests - at least or bugs covered by tests.

[vinzenz]

@vinzenz I know we are in time pressure, so in case you want to skip unit-tests now in this PR, please add at least TODO comment and create jira ticket. Regarding we are going to use multiple leapp repositories basically all the time, it will be better to have this field covered by tests - at least or bugs covered by tests.

While I can add a test, this is supposed to fix problems in leapp repository and these problems are somehow caused by pytest. I don't see this a big problem as the coverity of what is supported is covered and doesn't break.

I also made travis work again so finally it builds on travis - As they moved now to travis-ci.com instead of travis-ci.org for opensource code...

In another PR I am working on moving to github actions for unit tests anyway.

[Rezney]

The fix is working fine and does not seem to affect anything else.