schema: use separate package for capnpc-go and reflection usages

[zombiezen]

To avoid cyclic dependency problems and to make std/capnp/schema not special (i.e. include String() methods etc.), all of the packages in this repository that currently use std/capnp/schema should be made to use a new internal-only package that has the current generated code. This would not be a breaking change, as none of the public API surface exposes any of these types. The packages just use the schema package to implement the internals.