core: Add a [core] config section

I deemed it better to make core an extension (that cannot be disabled)
rather than add a special case throughout the config handling to make
it possible to have config section that doesn't belong to an extension.

This change is needed for #997.

Until #997 is completed, Mopidy will complain that core has now config
schema (because it is empty) and claim that core is disabled. This of
course has no practical effects.

mopidy/core/__init__.py

@@ -2,6 +2,7 @@
 
 # flake8: noqa
 from .actor import Core
+from .ext import Extension
 from .history import HistoryController
 from .library import LibraryController
 from .listener import CoreListener

mopidy/core/ext.conf

@@ -0,0 +1 @@
+[core]

mopidy/core/ext.py

@@ -0,0 +1,25 @@
+from __future__ import absolute_import, unicode_literals
+
+import os
+
+import mopidy
+from mopidy import config, ext
+
+
+class Extension(ext.Extension):
+
+    dist_name = 'Mopidy-Core'
+    ext_name = 'core'
+    version = mopidy.__version__
+
+    def get_default_config(self):
+        conf_file = os.path.join(os.path.dirname(__file__), 'ext.conf')
+        return config.read(conf_file)
+
+    def get_config_schema(self):
+        schema = super(Extension, self).get_config_schema()
+        del schema['enabled']  # core cannot be disabled
+        return schema
+
+    def setup(self, registry):
+        pass  # core has nothing to register

setup.py

@@ -34,6 +34,7 @@ def get_version(filename):
             'mopidy = mopidy.__main__:main',
         ],
         'mopidy.ext': [
+            'core = mopidy.core:Extension',
             'http = mopidy.http:Extension',
             'local = mopidy.local:Extension',
             'file = mopidy.file:Extension',