Populated models only when necessary in get_app_config[s].

Took this opportunity to change get_app[s] to only consider applications
containing a model module as that seems slightly more backwards compatible.

Since callers that care about models pass only_with_models_module=True,
this has very few consequences. Only AppCommand needed a change.

django/core/management/base.py

@@ -344,6 +344,11 @@ class AppCommand(BaseCommand):
         from django.core.apps import app_cache
         if not app_labels:
             raise CommandError('Enter at least one appname.')
+        # Populate models and don't use only_with_models_module=True when
+        # calling get_app_config() to tell apart missing apps from apps
+        # without a model module -- which can't be supported with the legacy
+        # API since it passes the models module to handle_app().
+        app_cache.populate_models()
         try:
             app_configs = [app_cache.get_app_config(app_label) for app_label in app_labels]
         except (LookupError, ImportError) as e: