Add InstanceGraph helpers: reachable/unreachable

Adds new APIs for querying sets of modules in an InstanceGraph:

  - modules: the set of all modules
  - reachableModules: set of modules reachable from the main/top
  - unreachableModules: set of modules not reachable from the main/top

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>