More tests.
* bzrlib/tests/test_plugins.py:
(TestPluginMixin.create_plugin_package.cleanup): Fix some random?
lazy import failure.
(TestLoadPluginAt.test_compiled_loaded,
TestLoadPluginAt.test_submodule_loading): Test that we load the
compiled version if available and that we can load submodules.
* bzrlib/plugin.py:
(_PluginImporter.load_module): Be ready for submodules.