Similar to the previous commit to include a `inputs_service` fixture
this does the same for `source.SourcesService` imports.
Note that we cannot easily share the helpers so we have to life with
a bit of very similar but duplicated code. To fix this we would have
to have a shared confftest.py that pytest can find. Which would mean
that we need to put the tests under a common dir that is reachable
via __init__.py files (which we currently not have because stages,
inputs etc do not have a __init__.py so python does not considers
them modules).
Similar to the `stage_module` fixture for stage tests this adds
a fixture to test sources modules of osbuild.
The code from `stage_module` and `sources_module` is similar and
could be combined but pytest makes it hard to do this without
having a shared root dir. Given that it's just four lines it
seems easier to just life with the tiny bit of code duplication.
