resolve packages for legacy plugins

Updated legacy plugin loader to handle pakcages if the PLUGINS entry
contains `.`. Also adds a test for it.

pelican/plugins/_utils.py

@@ -40,6 +40,11 @@ def list_plugins(ns_pkg=None):
 
 
 def load_legacy_plugin(plugin, plugin_paths):
+    if '.' in plugin:
+        # it is in a package, try to resolve package first
+        package, _, _ = plugin.rpartition('.')
+        load_legacy_plugin(package, plugin_paths)
+
     # Try to find plugin in PLUGIN_PATHS
     spec = importlib.machinery.PathFinder.find_spec(plugin, plugin_paths)
     if spec is None:

pelican/tests/dummy_plugins/normal_plugin/normal_submodule_plugin/subpackage/subpackage.py

@@ -0,0 +1,5 @@
+NAME = 'normal subpackage plugin'
+
+
+def register():
+    pass

pelican/tests/dummy_plugins/normal_plugin/normal_submodule_plugin/subplugin.py

@@ -0,0 +1,5 @@
+NAME = 'normal submodule plugin'
+
+
+def register():
+    pass

pelican/tests/test_plugins.py

@@ -116,6 +116,21 @@ class PluginTest(unittest.TestCase):
                 {'normal plugin'},
                 get_plugin_names(plugins))
 
+            # normal submodule/subpackage plugins
+            SETTINGS = {
+                'PLUGINS': [
+                    'normal_submodule_plugin.subplugin',
+                    'normal_submodule_plugin.subpackage.subpackage',
+                ],
+                'PLUGIN_PATHS': [self._NORMAL_PLUGIN_FOLDER]
+            }
+            plugins = load_plugins(SETTINGS)
+            self.assertEqual(len(plugins), 2, plugins)
+            self.assertEqual(
+                {'normal submodule plugin',
+                 'normal subpackage plugin'},
+                get_plugin_names(plugins))
+
             # namespace plugin short
             SETTINGS = {
                 'PLUGINS': ['ns_plugin']