application: Pass prelim opts to .find_plugins()

The configuration file and boolean to ignore configuration files can be
threaded through now that `.parse_preliminary_options_and_args()`
returns options and arguments.

src/flake8/api/legacy.py

@@ -33,7 +33,7 @@ def get_style_guide(**kwargs):
     )
     flake8.configure_logging(prelim_opts.verbose, prelim_opts.output_file)
     application.make_config_finder(prelim_opts.append_config, prelim_args)
-    application.find_plugins()
+    application.find_plugins(prelim_opts.config, prelim_opts.isolated)
     application.register_plugin_options()
     application.parse_configuration_and_cli([])
     # We basically want application.initialize to be called but with these

src/flake8/main/application.py

@@ -174,8 +174,8 @@ class Application(object):
                 self.option_manager.program_name, args, append_config
             )
 
-    def find_plugins(self):
-        # type: () -> None
+    def find_plugins(self, config_file, ignore_config_files):
+        # type: (Optional[str], bool) -> None
         """Find and load the plugins for this application.
 
         If :attr:`check_plugins`, or :attr:`formatting_plugins` are ``None``
@@ -183,12 +183,17 @@ class Application(object):
         instance. Given the expense of finding plugins (via :mod:`entrypoints`)
         we want this to be idempotent and so only update those attributes if
         they are ``None``.
+
+        :param str config_file:
+            The optional configuraiton file to override all other configuration
+            files (i.e., the --config option).
+        :param bool ignore_config_files:
+            Determine whether to parse configuration files or not. (i.e., the
+            --isolated option).
         """
         if self.local_plugins is None:
             self.local_plugins = config.get_local_plugins(
-                self.config_finder,
-                self.prelim_opts.config,
-                self.prelim_opts.isolated,
+                self.config_finder, config_file, ignore_config_files
             )
 
         sys.path.extend(self.local_plugins.paths)
@@ -367,7 +372,7 @@ class Application(object):
         )
         flake8.configure_logging(prelim_opts.verbose, prelim_opts.output_file)
         self.make_config_finder(prelim_opts.append_config, prelim_args)
-        self.find_plugins()
+        self.find_plugins(prelim_opts.config, prelim_opts.isolated)
         self.register_plugin_options()
         self.parse_configuration_and_cli(argv)
         self.make_formatter()

tests/unit/test_legacy_api.py

@@ -12,6 +12,8 @@ def test_get_style_guide():
     """Verify the methods called on our internal Application."""
     prelim_opts = argparse.Namespace(
         append_config=[],
+        config=None,
+        isolated=False,
         output_file=None,
         verbose=0,
     )
@@ -28,7 +30,7 @@ def test_get_style_guide():
     application.assert_called_once_with()
     mockedapp.parse_preliminary_options_and_args.assert_called_once_with([])
     mockedapp.make_config_finder.assert_called_once_with([], [])
-    mockedapp.find_plugins.assert_called_once_with()
+    mockedapp.find_plugins.assert_called_once_with(None, False)
     mockedapp.register_plugin_options.assert_called_once_with()
     mockedapp.parse_configuration_and_cli.assert_called_once_with([])
     mockedapp.make_formatter.assert_called_once_with()