Swift: merge codegen and cppcodegen

Python code was simplified, and now a `--generate` option can be used
to drive what can be generated.

The extractor pack creation now will use an internally generated
dbscheme. This should be the same as the checked in one, but doing so
allows `bazel run create-extractor-pack` and `bazel run codegen` to be
run independently from one another, while previously the former had to
follow the latter in case of a schema change. This is the change that
triggered the above simplification, as in order for the two dbscheme
files to be identical, the first `// generated` line had to state the
same generator script.

swift/codegen/generators/cppgen.py

@@ -5,7 +5,6 @@ import inflection
 from toposort import toposort_flatten
 
 from swift.codegen.lib import cpp, schema
-from swift.codegen.generators import generator
 
 
 def _get_type(t: str, trap_affix: str) -> str:
@@ -64,13 +63,8 @@ class Processor:
 
 
 def generate(opts, renderer):
+    assert opts.cpp_output
     processor = Processor({cls.name: cls for cls in schema.load(opts.schema).classes}, opts.trap_affix)
     out = opts.cpp_output
     renderer.render(cpp.ClassList(processor.get_classes(), opts.cpp_namespace, opts.trap_affix,
                                   opts.cpp_include_dir, opts.schema), out / f"{opts.trap_affix}Classes.h")
-
-
-tags = ("cpp", "schema")
-
-if __name__ == "__main__":
-    generator.run()