This is a prototype for keeping track of the path through the selection
process to a project. It is used to help dep ignore "stale" transitive
constraints: constraints that when created applied to a descendent but
should no longer apply now that that the project has moved to another
location in the dependency graph.

Questions:
* I put bmi on atomWithPackages because it already had a bmi method to
recreate the original bmi. Not sure if it's safe to live there
considering the comments on the original bmi function about avoiding
copys of the package list. So it may need to shift elsewhere or path
should be be split out from the bmi struct so that it can be attached
directly to an atom.
* Is a non-bimodal solve ever a possibility in dep (outside of the unit
tests?). I think we could drop "dep.isTransitive" in favor of using
bmi.path exclusively but the unit tests have a non-bimodal set of tests
that cause this to fail.