Add "Must Deduplicate" API (bp #2077) (#2081)

* Add DiGraph factory method and prettyTree

* New factory method enables direct construction of DiGraphs from edges
* DiGraph.prettyTree enables visualization of tree or multi-tree
  diagraphs

(cherry picked from commit 5ea82302)

* Add MustDeduplicateTransform

This enables marking modules as "must deduplicate". If modules marked as
such do not deduplicate, the transform will create error reports and
make suggestions as to why deduplication failed.

(cherry picked from commit 5903e6a3

)

Co-authored-by: Jack Koenig <koenig@sifive.com>