config: Switch code paths to use 'ConfigFileFinder.ignore_config_files'

Now that the `ConfigFileFinder` has the `.ignore_config_files`
attribute, switch the relevant code paths to utilize this public
attribute.

Tests have been updated to either construct `ConfigFileFinder` or mock
the object appropriately.

src/flake8/options/config.py

@@ -307,7 +307,7 @@ class MergedConfigParser(object):
         :rtype:
             dict
         """
-        if isolated:
+        if self.config_finder.ignore_config_files:
             LOG.debug(
                 "Refusing to parse configuration files due to user-"
                 "requested isolation"
@@ -344,7 +344,7 @@ def get_local_plugins(config_finder, cli_config=None, isolated=False):
         flake8.options.config.LocalPlugins
     """
     local_plugins = LocalPlugins(extension=[], report=[], paths=[])
-    if isolated:
+    if config_finder.ignore_config_files:
         LOG.debug(
             "Refusing to look for local plugins in configuration"
             "files due to user-requested isolation"

tests/integration/test_aggregator.py

@@ -43,7 +43,8 @@ def test_aggregate_options_when_isolated(optmanager):
     """Verify we aggregate options and config values appropriately."""
     arguments = ['flake8', '--select', 'E11,E34,E402,W,F',
                  '--exclude', 'tests/*']
-    config_finder = config.ConfigFileFinder('flake8', [])
+    config_finder = config.ConfigFileFinder(
+        'flake8', [], ignore_config_files=True)
     optmanager.extend_default_ignore(['E8'])
     options, args = aggregator.aggregate_options(
         optmanager, config_finder, None, True, arguments)

tests/unit/test_get_local_plugins.py

@@ -21,6 +21,7 @@ def test_get_local_plugins_uses_cli_config():
     config_obj = mock.Mock()
     config_finder = mock.MagicMock()
     config_finder.cli_config.return_value = config_obj
+    config_finder.ignore_config_files = False
     config_obj.get.return_value = ''
 
     config.get_local_plugins(config_finder, cli_config='foo.ini')

tests/unit/test_merged_config_parser.py

@@ -155,6 +155,7 @@ def test_parse_isolates_config(optmanager):
 def test_parse_uses_cli_config(optmanager):
     """Verify behaviour of the parse method with a specified config."""
     config_finder = mock.MagicMock()
+    config_finder.ignore_config_files = False
     parser = config.MergedConfigParser(optmanager, config_finder)
 
     parser.parse(cli_config='foo.ini')