Dependency resolution via pomegranate/aether.

`resolve-source-deps` produces the resolved tree (Maven conflict resolution has
run, each coordinate pinned to one version); `expand-dep-hierarchy` re-expands
it into the unresolved tree where the same library may appear in many branches
at different versions. These two trees feed the two inlining modes.

Walking and ordering of dependency trees.

A dependency tree here is a nested map of `[name version] -> subtree`. This
namespace provides depth-first walks for both modes (`walk-dep-tree` for
unresolved, `walk-ordered-deps` for resolved), tree expansion
(`walk&expand-deps`), subtree eviction, and topological ordering.