cmd/go/internal/modload: consolidate buildList and associated functions into one file
    
    Change-Id: I310c37c7f0ce5581f07cf6e27d1f6361d03b92ef
    Reviewed-on: https://go-review.googlesource.com/c/go/+/244077
    Run-TryBot: Bryan C. Mills <bcmills@google.com>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: Michael Matloob <matloob@golang.org>
    Reviewed-by: Jay Conrod <jayconrod@google.com>