Enable reflecting of ext types.

Reflect any types appearing in the extensions.

We still hide built-in modules like `cfg`, `sys`, and `schema` from
reflection to ORM. This also means types derived from these or links to
them.

Also clean up generated imports.

Author: vpetrovykh

gel/orm/introspection.py

@@ -50,7 +50,11 @@
     and
     not .internal
     and
-    not re_test('^(std|cfg|sys|schema)::', .name);
+    not .from_alias
+    and
+    not re_test('^(std|cfg|sys|schema)::', .name)
+    and
+    not any(re_test('^(cfg|sys|schema)::', .ancestors.name));
 '''
 
 MODULE_QUERY = '''

gel/orm/sqla.py

@@ -124,9 +124,8 @@ def init_module(self, mod, modules):
             try:
                 self.out = f
                 self.write(MODELS_STUB)
-                relimport = '.' * len(dirpath)
-                self.write(f'from {relimport}._sqlabase import Base')
-                self.write(f'from {relimport}._tables import *')
+                self.write(f'from {self.basemodule}._sqlabase import Base')
+                self.write(f'from {self.basemodule}._tables import *')
                 yield f
             finally:
                 self.out = None
@@ -201,11 +200,11 @@ def render_models(self, spec):
                 self.render_link_table(rec)
 
         for mod, maps in modules.items():
-            with self.init_module(mod, modules):
-                if not maps:
-                    # skip apparently empty modules
-                    continue
+            if not maps:
+                # skip apparently empty modules
+                continue
 
+            with self.init_module(mod, modules):
                 link_objects = sorted(
                     maps.get('link_objects', {}).values(),
                     key=lambda x: x['name'],