BZR_PLUGINS_AT should use packages properly to handle relative imports.

* bzrlib/tests/test_plugins.py:
(TestPluginMixin._unregister_plugin_submodule): Be more robust on
errors.

* bzrlib/plugin.py:
(_PluginImporter.load_module): Packages should be loaded
differently, trying to load the __init__.py file explicitly
doesn't work.