txemail: make gophermail types unexported
Created by: keegancsmith
We unnecessarily exposed gophermail types in our txemail/... packages. This change moves all use of gophermail into just the txemail package. Additionally it avoids using it in the exported API.
This makes it easier to change the package we use for email. Currently gophermail is archived and unmaintained.
Part of https://github.com/sourcegraph/sourcegraph/issues/10703