Push down extra config file path normalization into main config handling

Move the path normalization for extra configuration file paths down into
the main `config` module where other path normalization occurs.

This also guarantees that the call to `utils.normalize_paths()` is given
a sequence, instead of a potential `None` value.

src/flake8/main/application.py

@@ -158,13 +158,10 @@ class Application(object):
     def make_config_finder(self):
         """Make our ConfigFileFinder based on preliminary opts and args."""
         if self.config_finder is None:
-            extra_config_files = utils.normalize_paths(
-                self.prelim_opts.append_config
-            )
             self.config_finder = config.ConfigFileFinder(
                 self.option_manager.program_name,
                 self.prelim_args,
-                extra_config_files,
+                self.prelim_opts.append_config,
             )
 
     def find_plugins(self):

src/flake8/options/config.py

@@ -28,11 +28,7 @@ class ConfigFileFinder(object):
         """
         # The values of --append-config from the CLI
         extra_config_files = extra_config_files or []
-        self.extra_config_files = [
-            # Ensure the paths are absolute paths for local_config_files
-            os.path.abspath(f)
-            for f in extra_config_files
-        ]
+        self.extra_config_files = utils.normalize_paths(extra_config_files)
 
         # Platform specific settings
         self.is_windows = sys.platform == "win32"