pkg: make $id in tasks a reference to a hidden field
As suggested by Paul and Marcel, hidden fields are unique to a package,
so unlike regular fields like `$id` they cannot be replaced by
a field from a different package or a field from unmarshalled data.
For each of the task definitions, keep `$id` around for backwards
compatibility, but make it a reference to the new `_id`.
Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I8d1d8ba765f782e2cb9d3f70612c7834fef53fa5
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1214986
Reviewed-by: Paul Jolly <paul@myitcv.io>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>