osbuild: add new testutil.imports module to help test stages

This commit adds `osbuild.testutil.imports.import_module_from_path`
that can be used to import arbitrary python source files. This
allows importing files from the stages directory that have a
non python friendly filename like `org.osbuild.kickstart`.

osbuild/testutil/__init__.py

@@ -0,0 +1,3 @@
+"""
+Test related utilities
+"""

osbuild/testutil/imports.py

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+#!/usr/bin/python3
+"""
+Import related utilities
+"""
+import importlib
+from types import ModuleType
+
+
+def import_module_from_path(fullname, path: str) -> ModuleType:
+    """import_module_from_path imports the given path as a python module
+
+    This helper is useful when importing things that are not in the
+    import path or have invalid python import filenames, e.g. all
+    filenames in the stages/ dir of osbuild.
+
+    Keyword arguments:
+    fullname -- The absolute name of the module (can be arbitrary, used on in ModuleSpec.name)
+    path     -- The full path to the python file
+    """
+    loader = importlib.machinery.SourceFileLoader(fullname, path)
+    spec = importlib.util.spec_from_loader(loader.name, loader)
+    if spec is None:
+        # mypy warns that spec might be None so handle it
+        raise ImportError(f"cannot import {fullname} from {path}, got None as the spec")
+    mod = importlib.util.module_from_spec(spec)
+    loader.exec_module(mod)
+    return mod

test/mod/test_testutil_imports.py

@@ -0,0 +1,17 @@
+#
+# Tests for the 'osbuild.util.testutil' module.
+#
+import os.path
+import tempfile
+
+import pytest
+
+from osbuild.testutil.imports import import_module_from_path
+
+
+canary = "import-went-okay"
+
+
+def test_import_module_from_path_happy():
+    mod = import_module_from_path("myself", __file__)
+    assert mod.canary == "import-went-okay"