internal/mod/modload: retain source field when tidying
The code to copy the module file from the old version
did not include the source field.
Fixes #3056.
Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I50505b0c9abf52023e7b4cff73a26cd3a22f7362
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1193708
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>