plugins.handlers: fix incorrect PyFilePlugin construction

Two things:

1. `submodule_search_locations` is expected to be a list, according to
   Python's documentation for `importlib.machinery.ModuleSpec`.
2. The new `module` being inserted into `sys.modules` before checking if
   its spec had a valid `loader` bothered me, so I reordered the steps.

sopel/plugins/handlers.py

@@ -471,7 +471,7 @@ def __init__(self, filename):
             spec = importlib.util.spec_from_file_location(
                 name,
                 os.path.join(filename, '__init__.py'),
-                submodule_search_locations=filename,
+                submodule_search_locations=[filename],
             )
         else:
             raise exceptions.PluginError('Invalid Sopel plugin: %s' % filename)
@@ -487,9 +487,9 @@ def __init__(self, filename):
 
     def _load(self):
         module = importlib.util.module_from_spec(self.module_spec)
-        sys.modules[self.name] = module
         if not self.module_spec.loader:
             raise exceptions.PluginError('Could not determine loader for plugin: %s' % self.filename)
+        sys.modules[self.name] = module
         self.module_spec.loader.exec_module(module)
         return module