More portable support for "Add project by file" and "Upgrade & add workspace" commands if `find` and `ln` are unavailable

[abathur]

Constellation currently assumes the find and ln will be available. It doesn't test for their availability or implement fallbacks.

• "Add project by file" (only requires find)
  • Disable command if find is unavailable
  • If it can be done with tolerable performance, implement a platform-independent no-dependency fallback if the command is unavailable
• "Upgrade & add workspace"
  • Disable command if either find or ln are unavailable
  • If it can be done with tolerable performance, implement a platform-independent no-dependency fallback if the command is unavailable