sg: Remove go.mod

Created by: efritz

Proposal: Remove the go.mod/go.sum file from ./dev/sg so that sg can import things from internal/ in this repository. I talked with @mrnugget on slack and he didn't have a problem with this as long as install and binary releases still work. I built locally and with the install script (seems fine), but haven't checked binary release workflow yet (not sure how to check yet).

Currently I'm working on rewriting our db migration infrastructure. We will no longer have a dependency on a third-party library, which means we either need to move database connection, migration, and basestore utilities into lib/, which seems to encourage direct access to our database from non-sg utilities (NOT INTENTIONAL), or have sg subprocess something inside of cmd/migrator, which seems a bit clunky.

Combining the dependencies/import visibility of sg makes sense as it's a blessed app that should be able to do nasty things in development environments we wouldn't want to encourage or perform on live instances. This feels like the most correct solution to me today.