application: Change to argparse.ArgumentParser for preliminary parsing

Now that preliminary options are registered with the preliminary parser
on the `Application`, leverage that for parsing known options.

This important change removes the `Application.option_manager` from
being responsible for pre-configuration parsing and the workarounds
needed in the `Application.parse_preliminary_options_and_args()` to
account for the fact that `Application.option_manager` was aware of
*all* options, not just the options necessary for pre-configuration
loading.  A following commit will address removing these workarounds.

src/flake8/main/application.py

@@ -138,7 +138,7 @@ class Application(object):
         except ValueError:
             pass
 
-        opts, args = self.option_manager.parse_known_args(args)
+        opts, args = self.prelim_arg_parser.parse_known_args(args)
         # parse_known_args includes unknown options as args
         args = [a for a in args if not a.startswith("-")]
         return opts, args

tests/unit/test_application.py

@@ -93,9 +93,10 @@ def test_returns_specified_plugin(application):
 def test_prelim_opts_args(application):
     """Verify we get sensible prelim opts and args."""
     opts, args = application.parse_preliminary_options_and_args(
-        ['flake8', '--foo', '--verbose', 'src', 'setup.py', '--statistics'])
+        ['--foo', '--verbose', 'src', 'setup.py', '--statistics'])
 
-    assert opts.statistics
+    assert not hasattr(opts, 'foo')
+    assert not hasattr(opts, 'statistics')
     assert opts.verbose
     assert args == ['src', 'setup.py']