Use mvifdiff and stamp files to track whether a .gox file has changed.
    When package A depends on package B, and we rebuild package B, only
    rebuild package A if package B's .gox file changes.  This is safe
    because when package A imports package B it only reads package B's .gox
    file.  This means that changes that do not affect export information
    will not cause dependent packages to be recompiled.
    
    Reviewed-on: https://go-review.googlesource.com/32476

From-SVN: r241742