Avoid stat-ing all packages at startup. (#7587)

- FastPackageChecker was being called at startup every time Spack runs,
  which takes a long time on networked filesystems.  Startup was taking
  5-7 seconds due to this call.

- The checker was intended to avaoid importing all packages (which is
  really expensive) when all it needs is to stat them.  So it's only
  "fast" for parts of the code that *need* it.

- This commit makes repositories instantiate the checker lazily, so it's
  only constructed when needed.