Previously we handled all of the config sources directly within the main
function. We're going to make CLI config loading more complex shortly, so
having this encapsulated in its own function will avoid creating even more
clutter inside the main function.

Along the way here we also switch from using native Go "error" to using
tfdiags.Diagnostics, so that we can potentially issue warnings here too
in future, and so that we can return multiple errors.