Continuing the massacre, remove ParseModHandle, and Mod*Handle, from the
source API.
Notably, having the snapshot available means we can simplify the go
command invocation paths a lot.
Change-Id: Ief4ef41e42f93d653f719a230004861e5e1ef70b
Reviewed-on: https://go-review.googlesource.com/c/tools/+/244769
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>