Interactive CLI option for move_to_folder

[schoblaska]

Adds an interactive CLI option for the move_to_folder command added in #136.

Both move_to_folder and move_to_parent are nested under a top-level CLI option "Move a pack."

If you attempt to move a pack into a folder that happens to be a pack itself, you get a warning message and are given the chance to use the move_to_parent command instead.

$ bin/packs
Hello! What would you like to do? (Press ↑/↓ arrow to move, Enter to select
nd letters to filter)
  1. Create a new pack
  2. Move files
‣ 3. Move a pack
  4. Add a dependency
  5. Get info on one or more packs
  6. Query violations about a pack
  7. Make files or directories public
  8. Rename a pack
  9. Run bin/packwerk check
  10. Run bin/packwerk update
  11. Run bin/packwerk validate (detects cycles)
  12. Lint packs/*/package.yml for one or more packs

Hello! What would you like to do? Move a pack
What do you want to do? (Press ↑/↓ arrow to move and Enter to select)
‣ Move a child pack to be nested under a parent pack
  Move a pack to a folder that is not a pack
  
$ bin/packs
Hello! What would you like to do? Move a pack
What do you want to do? Move a pack to a folder that is not a pack
Please select the pack that you want to move packs/foo
What directory do you want to move packs/foo to? packs/bar
The directory packs/bar contains a pack. Add foo as a dependency?
(Press ↑/↓ arrow to move and Enter to select)
‣ Yes
  No

Fixes #135

[perryqh]

Really solid PR! I just have a question about testing.