repos: lookup <buildin>, /{etc,usr/share}/image-builder/repositories

This commit switches the default repositories to use the build-in
ones from images PR#1112. It is still possible to override using
`/etc/image-builder` and `/usr/share/image-builder` or `--datadir`.

This is implicitly tested via the container test that no longer
includes the files from osbuild-composer in the container.

test/conftest.py

@@ -0,0 +1,17 @@
+import subprocess
+
+import pytest
+
+
+# XXX: copied from bib
+@pytest.fixture(name="build_container", scope="session")
+def build_container_fixture():
+    """Build a container from the Containerfile and returns the name"""
+
+    container_tag = "image-builder-cli-test"
+    subprocess.check_call([
+        "podman", "build",
+        "-f", "Containerfile",
+        "-t", container_tag,
+    ])
+    return container_tag