Improve formatting in `raco pkg show`, and add single-package show.

* `raco pkg show typed-racket` now shows just the "typed-racket" pkg.

* `raco pkg show --rx typed-racket` shows all packages that match the
  regular expression "typed-racket".

* `raco pkg show` now only shows the first 8 characters of checksums
  unless you provide the `--full-checksum` argument.