Improve `spack find` output in environments (#42334)

This adds some improvements to `spack find` output when in environments based
around some thoughts about what users want to know when they're in an env.

If you're working in an enviroment, you mostly care about:
* What are the roots
* Which ones are installed / not installed
* What's been added that still needs to be concretized

So, this PR adds a couple tweaks to display that information more clearly:

- [x] We now display install status next to every root. You can easily see
      which are installed and which aren't.

- [x] When you run `spack find -l` in an env, the roots now show their concrete
      hash (if they've been concretized). They previously would show `-------`
      (b/c the root spec itself is abstract), but showing the concretized root's
      hash is a lot more useful.

- [x] Newly added/unconcretized specs still show `-------`, which now makes more
      sense, b/c they are not concretized.

- [x] There is a new option, `-r` / `--only-roots` to *only* show env roots if
      you don't want to look at all the installed specs.

- [x] Roots in the installed spec list are now highlighted as bold. This is
      actually an old feature from the first env implementation , but various
      refactors had disabled it inadvertently.

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