Fix cyclic import in the module API (#11180)

Introduced in #10548

See https://github.com/matrix-org/synapse-email-account-validity/runs/3979337154?check_suite_focus=true for an example of a module's CI choking over this issue.

changelog.d/11180.feature

@@ -0,0 +1 @@
+Port the Password Auth Providers module interface to the new generic interface.

synapse/handlers/auth.py

@@ -62,7 +62,6 @@
 from synapse.http.site import SynapseRequest
 from synapse.logging.context import defer_to_thread
 from synapse.metrics.background_process_metrics import run_as_background_process
-from synapse.module_api import ModuleApi
 from synapse.storage.roommember import ProfileInfo
 from synapse.types import JsonDict, Requester, UserID
 from synapse.util import stringutils as stringutils
@@ -73,6 +72,7 @@
 from synapse.util.threepids import canonicalise_email
 
 if TYPE_CHECKING:
+    from synapse.module_api import ModuleApi
     from synapse.rest.client.login import LoginResponse
     from synapse.server import HomeServer
 
@@ -1818,7 +1818,9 @@ def load_legacy_password_auth_providers(hs: "HomeServer") -> None:
 
 
 def load_single_legacy_password_auth_provider(
-    module: Type, config: JsonDict, api: ModuleApi
+    module: Type,
+    config: JsonDict,
+    api: "ModuleApi",
 ) -> None:
     try:
         provider = module(config=config, account_handler=api)