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'.