Swift: some restructuring of codegen

Loading of the schema and dbscheme has been moved to a separate
`loaders` package for better separation of concerns.

swift/codegen/generators/cppgen.py

@@ -17,6 +17,7 @@ import typing
 import inflection
 
 from swift.codegen.lib import cpp, schema
+from swift.codegen.loaders import schemaloader
 
 
 def _get_type(t: str, add_or_none_except: typing.Optional[str] = None) -> str:
@@ -90,7 +91,7 @@ class Processor:
 
 def generate(opts, renderer):
     assert opts.cpp_output
-    processor = Processor(schema.load_file(opts.schema))
+    processor = Processor(schemaloader.load_file(opts.schema))
     out = opts.cpp_output
     for dir, classes in processor.get_classes().items():
         renderer.render(cpp.ClassList(classes, opts.schema,