Finalize the Go API for pgroll

[ryanslade]

At the moment all of our packages are available in the pkg package. Since pgroll is designed to be used as a library, we may want to move some of these packages into an internal package so that we reduce the scope of what can be imported externally.

This will clean up our API and make it easier to support in the long term.

[ryanslade]

TODO: @ryanslade to provide a more concrete proposal here.

WIP

• Create top-level internal package (same level as pkg)
• Move pkg/testutils into internal
• Move pkg/jsonschema into internal

[gulcin]

We'd like this issue to be ready for v1.0. We can discuss remaining things in it later.