Move htools backends to a separate directory

Five modules under the HTools/ directories are backend
implementations, so let's move them to a separate directory, to more
clearly show the hierarchy. I wanted to do this for a while, but
merging between branches is always an issue, so let's do it know since
we have an opportunity.

This patch contains the actual renames, the required changed module
names, imports, etc., but no other changes.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>