cmd/go: add support for godebug lines in go.mod and go.work
The fact that the go line sets both the language version and the
GODEBUG compatibility version can be a problem, especially since
the go line is also required to be ≥ the go lines of any required
dependency modules.
This change adds a new 'godebug' line to go.mod and go.work
to allow setting the GODEBUG values for the entire module.
It also adds a new meta-value default=go1.21 that means
take the defaults from Go 1.21 no matter what the go line says.
These were discussed in proposal #65573.
Fixes #65573.
Change-Id: I91746322a10178370ed1015ce5278372a024c824
Reviewed-on: https://go-review.googlesource.com/c/go/+/584476
Auto-Submit: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Sam Thanawalla <samthanawalla@google.com> Reviewed-by: Michael Matloob <matloob@golang.org>