The curl source is the only source left that uses "transform". And
here the name is very generic but in fact we only do a single thing:
we add secrets for subscriptions for for mtls to the download.
So rename to make it clear what this is all about.
The current `make_container()` helper is a bit silly (which is
entirely my fault). It requires a container tag as input but all
tests end up creating a random number for this input. So instead
just remove the input and return the container_id from the podman
build in the contextmanager and use that.
When an images does not exist just return `False` instead of
raising a RuntimeError. If anything else goes wrong (unknown
output or hash mismatch) keep the RuntimeError as this is an
unexpected exception.
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.
