Add ability to lookup via CLI.

    python -m intersphinx_registry.lookup [package names]

Not direction -m intersphinx_registry in case we want to add
functionalities later.

.github/workflows/tests.yml

@@ -29,7 +29,7 @@ jobs:
           python-version: ${{ matrix.python }}
       - run:
           pip install -ve .[tests]
-          pip install mypy
+          pip install mypy sphinx
       - run:
           pytest tests
       - run:

intersphinx_registry/lookup.py

@@ -0,0 +1,19 @@
+import sys
+
+from intersphinx_registry import get_intersphinx_mapping
+import logging as _logging
+from sphinx.ext.intersphinx import inspect_main
+
+if len(sys.argv) < 2:
+    sys.exit("Usage: python -m intersphinx_registry.lookup [packages...]")
+
+packages = set(sys.argv[1:])
+
+urls = [
+    u[0] + (u[1] if u[1] else "objects.inv")
+    for u in get_intersphinx_mapping(packages=packages).values()
+]
+
+
+_logging.basicConfig()
+raise SystemExit(inspect_main(urls))