cmd/go: look for tool build ID before hashing entire file
cmd/go: look for tool build ID before hashing entire file

Also fix the key used to store the ID.

This is a significant speedup in cmd/go run time.

Change-Id: Ie4c2f945872204fe24118d2adb2163f498d4394e
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/194257
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend