test: add sources_module fixture for sources unit tests

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.

sources/test/conftest.py

@@ -0,0 +1,21 @@
+import os
+import pathlib
+from types import ModuleType
+
+import pytest
+
+from osbuild.testutil.imports import import_module_from_path
+
+
+@pytest.fixture
+def sources_module(request: pytest.FixtureRequest) -> ModuleType:
+    """sources_module is a fixture that imports a stage module by its name
+    defined in SOURCES_NAME in the test module.
+    """
+    if not hasattr(request.module, "SOURCES_NAME"):
+        raise ValueError("sources_module fixture must be used in a test module that defines SOURCES_NAME")
+
+    sources_name = request.module.SOURCES_NAME
+    caller_dir = pathlib.Path(request.node.fspath).parent
+    module_path = caller_dir.parent / sources_name
+    return import_module_from_path("sources", os.fspath(module_path))