…lic curation (apple#898)

* Add functions to operate on directed graphs

* Update topic graph to use sequences for graph traversal

* Avoid computing full paths to determine if a symbol is manually curated

* Fix an infinite recursion determining the paths to a node when there's cyclic curation

rdar://126974763

* Avoid computing all paths when the caller only needs the shortest path

* Avoid computing all paths when the caller only needs the roots/leaves

* Avoid computing all paths when the caller only needs know if a certain node is encountered

* Avoid computing all paths when the caller only needs to visit each reachable node once

* Rename 'pathsTo(_:options:)' to 'finitePaths(to:options:)

* Fix another infinite recursion when pages are curated in cycles

rdar://126974763

* Fix correctness issue where symbol has two auto curated parents

* Address code review feedback:
- Rename neighbors parameter to edges for DirectedGraph initializer
- Rename GraphPathIterator and move it to DirectedGraph+Paths file
- Add convenience properties for topic graph directed graph "views
- Elaborate on breadcrumbs path documentation and implementation comments
- Elaborate on graph cycle documentation with concrete examples
- Fix missing edge in directed graph test data
- Use preconditionFailure for topic graph node that should always exist
- Add additional graph cycle tests

* Explicitly exit (trap) if trying to dump a topic graph with cyclic paths