Fix bug #552922 by controlling which files can be used to load a plugin.
* bzrlib/tests/test_plugins.py: (TestLoadPluginAt.test_loading_from___init__): Add one more test when loading from a file.
* bzrlib/plugin.py: (_PluginImporter.load_module): Don't use _find_module as its contract is also to find the plugin name. We *know* the plugin name here.