cmd/go: ignore build tags when 'go get' modifies build list

In module mode, 'go get' should not consider build constraints when
loading packages in order to modify the module graph. With this
change, 'go get' considers all build tags to be true except for
"ignore" and malformed build constraint expressions.

When 'go get' builds packages, it still applies build constraints for
the target platform.

Fixes #32345

Change-Id: I6dceae6f10a5185870537de730b36292271ad124
Reviewed-on: https://go-review.googlesource.com/c/go/+/179898


Reviewed-by: Bryan C. Mills <bcmills@google.com>