cmd/cue: fix mod edit --source self
The `cue mod edit --source self` wasn't tested and the logic was wrong, so fix it.
Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I809c466c7c76efde34046b83f70c58760dd7f3fd
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1193381
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>