plugins: support multiple sopel.modules submodule search locations

I'll be the first to admit, this doesn't actually *change* the built-in
plugins that are enumerated. But that was also the point: It *should*
give the same list as before.

Doing this is all about code hygiene. I just don't like blindly pulling
the `[0]`th element of a list and ignoring the rest, even if that list
only has a single entry. So the code to `find_internal_plugins()` now
accounts for the possibility that there could be multiple places it has
to search for those, and deduplicates the names found before trying to
load them.

Author: dgw

sopel/plugins/__init__.py

@@ -68,8 +68,14 @@ def find_internal_plugins():
     include the ``coretasks`` plugin.
     """
     modules = importlib.util.find_spec('sopel.modules')
-    submodule_dir = modules.submodule_search_locations[0]  # only one expected
-    for name, _ in _list_plugin_filenames(submodule_dir):
+    plugin_list = itertools.chain(
+        itertools.chain.from_iterable([
+            _list_plugin_filenames(path)
+            for path in modules.submodule_search_locations
+        ])
+    )
+
+    for name, _ in set(plugin_list):
         yield handlers.PyModulePlugin(name, 'sopel.modules')