Spec traversal: add option for topological ordering (#33817)

Spec traversals can now specify a topological ordering. A topologically-
ordered traversal with input specs X1, X2... will

* include all of X1, X2... and their children
* be ordered such that a given node is guaranteed to appear before any
  of its children in the traversal

Other notes:

* Input specs can be children of other input specs (this is useful if
  a user specifies a set of specs to uninstall: some of those specs
  might be children of others)
* `direction="parents"` will produce a reversed topological order
  (children always come before parents).
* `cover="edges"` will generate a list of edges L such that (a) input
  edges will always appear before output edges and (b) if you create
  a list with the destination of each edge in L the result is
  topologically ordered

0 files changed, 0 insertions, 0 deletions