Change HomeServer definition to work with typing.

Duplicating function signatures between server.py and server.pyi is
silly. This commit changes that by changing all `build_*` methods to
`get_*` methods and changing the `_make_dependency_method` to work work
as a descriptor that caches the produced value.

There are some changes in other files that were made to fix the typing
in server.py.

synapse/handlers/oidc_handler.py

@@ -14,7 +14,7 @@
 # limitations under the License.
 import json
 import logging
-from typing import Dict, Generic, List, Optional, Tuple, TypeVar
+from typing import TYPE_CHECKING, Dict, Generic, List, Optional, Tuple, TypeVar
 from urllib.parse import urlencode
 
 import attr
@@ -39,9 +39,11 @@ from synapse.http.server import respond_with_html
 from synapse.http.site import SynapseRequest
 from synapse.logging.context import make_deferred_yieldable
 from synapse.push.mailer import load_jinja2_templates
-from synapse.server import HomeServer
 from synapse.types import UserID, map_username_to_mxid_localpart
 
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
 logger = logging.getLogger(__name__)
 
 SESSION_COOKIE_NAME = b"oidc_session"
@@ -91,7 +93,7 @@ class OidcHandler:
     """Handles requests related to the OpenID Connect login flow.
     """
 
-    def __init__(self, hs: HomeServer):
+    def __init__(self, hs: "HomeServer"):
         self._callback_url = hs.config.oidc_callback_url  # type: str
         self._scopes = hs.config.oidc_scopes  # type: List[str]
         self._client_auth = ClientAuth(