This introduces two namespaces MAIN and TYPE to allow top-level symbol
collisions for type methods.

For example, `backend` is a reserved keyword but `backend.resolve` is a
type method. It is impossible to declare such a method symbol because it
would require its parent `backend` symbol to be a type symbol.

There would be a conflict between two `backend` symbols of different
kinds. The same would apply if the BLOB type had methods and vmod_blob
was imported.

At this point only the MAIN namespace is used but the split enables the
symbolization of types and type methods down the line.