Revert "bad merge diffs"

This reverts commit 7946be9f9b75fa43e700f519cb0caa07a352aac9.
Revert "one weird hack to maybe have better messages for nonexistent packages"

This reverts commit 75df1097e06dc7faed95e3f83d357ff3de63872f.
track ancestry as chain, not just root

switch constraintAncestor to be a nested mori.hash_map/mori.list
Also use mori for choices

Now there's no _.clone in resolver.js!
A more specific error for nonexistent package

This helps with #2410 though that particular case doesn't work since
paths are explored in a bad order.
Prefer to explore local packages first

(Or other packages where there is literally one version in existence.)
Merge branch 'constraint-solver-errors-ancestor' into release-0.9.0

Improves the #2410 fix to (at least for the particular case) be a little
less hacky: it's now OK if bad packages exist somewhere in the graph as
long as there's a way to avoid them.
Refactor ConstraintSolver.Resolver

Factor out the "state" into its own class, ResolverState.  The big
difference from the previous state object: it actually explicitly tracks
the set of potential UnitVersions for every active dependency.  This
essentially replaces the DependencyList class.

Because we always know exactly how many options there are for a given
dependency, we can both generalize and simplify the "propagate
transitive exact deps" optimization.  That optimization only worked on
"foo@=1.2.3" dependencies, which meant it didn't apply in any other
situation where there was only one possible package to choose. But there
are a whole lot of other situations like that: local packages, packages
that just don't have many versions, packages that already have a lot of
constraints applied to them, etc.  By tracking the set of potential
alternatives, we can just make sure to always expand 1-alternative units
first. We also maintain the aspect of the optimization where we don't
need to call the cost function until we've actually gotten to a state
with multiple neighbors.

This keeps #2410 fixed as well.

I've removed the constraintAncestor support as part of this refactoring,
so some error messages may be worse than they were before. But this
should set me up pretty well to improve error messages tomorrow.
Merge branch 'release-0.9.0' into devel