[release-branch.go1.17] cmd/go: update for //go:build lines

cmd/go has its own //go:build evaluator, which is needed for
patterns like 'all'. The code is a modified copy of some unexported
routines from the go/build package. Update it by copying those
again and re-modifying them. The modifications are primarily the new
func eval and also ignoring errors.

This CL will need to be backported to Go 1.17, or else Go 1.17
will break when faced with certain //go:build-only repos during
'go list all' or 'go mod tidy'.

For #41184.
Fixes #49198.

Change-Id: Ie0fe3caa8d49004935ecd76d7977f767fe50e317
Reviewed-on: https://go-review.googlesource.com/c/go/+/359355


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/359404