__main__.py: Adding private entry point so we can run BuildStream without installing

This is needed to build docs without installing BuildStream.

buildstream/__main__.py

+##################################################################
+#                      Private Entry Point                       #
+##################################################################
+#
+# This allows running the cli when BuildStream is uninstalled,
+# as long as BuildStream repo is in PYTHONPATH, one can run it
+# with:
+#
+#    python3 -m buildstream [program args]
+#
+# This is used when we need to run BuildStream before installing,
+# like when we build documentation.
+#
+if __name__ == '__main__':
+    # pylint: disable=no-value-for-parameter
+    from ._frontend.cli import cli
+    cli()