Re-use ServerApp.config_file_paths for consistency (jupyter-server#715)

* Use app.config_file_path to find server extensions

This property in the base class already has the same logic to add config_dir to jupyter_config_path

JupyterHub overrides this to have slightly different logic, and the same should apply here

* extensions should inherit parent's config_file_path

base ServerApp should have total control over config file loading

jupyter_server/extension/application.py

@@ -162,6 +162,11 @@ class method. This method can be set as a entry_point in
     def _default_open_browser(self):
         return self.serverapp.config["ServerApp"].get("open_browser", True)
 
+    @property
+    def config_file_paths(self):
+        """Look on the same path as our parent for config files"""
+        return self.serverapp.config_file_paths
+
     # The extension name used to name the jupyter config
     # file, jupyter_{name}_config.
     # This should also match the jupyter subcommand used to launch

jupyter_server/serverapp.py

@@ -104,7 +104,6 @@
     base_flags,
     base_aliases,
 )
-from jupyter_core.paths import jupyter_config_path
 from jupyter_client import KernelManager
 from jupyter_client.kernelspec import KernelSpecManager
 from jupyter_client.session import Session
@@ -2154,11 +2153,7 @@ def find_server_extensions(self):
         # This enables merging on keys, which we want for extension enabling.
         # Regular config loading only merges at the class level,
         # so each level clobbers the previous.
-        config_paths = jupyter_config_path()
-        if self.config_dir not in config_paths:
-            # add self.config_dir to the front, if set manually
-            config_paths.insert(0, self.config_dir)
-        manager = ExtensionConfigManager(read_config_path=config_paths)
+        manager = ExtensionConfigManager(read_config_path=self.config_file_paths)
         extensions = manager.get_jpserver_extensions()
 
         for modulename, enabled in sorted(extensions.items()):