Adjusts for the move from golang.org/x/tools/go/types and .../go/exact
to go/types and go/constant in the main repository.

Change-Id: I0da7248c540939e3e9b09c915b0a296937f1be73
Reviewed-on: https://go-review.googlesource.com/12284Reviewed-by: Andrew Gerrand <adg@golang.org>