Binary caching: use full hashes (#19209)

* "spack install" now has a "--require-full-hash-match" option, which
  forces Spack to skip an available binary package when the full hash
  doesn't match. Normally only a DAG-hash match is required, which
  ensures equivalent Specs, but does not account for changing logic
  inside the associated package.
* Add a local binary cache index which tracks specs that have a binary
  install available in a remote binary cache. It is updated with
  "spack buildcache list" or for a given spec when a binary package
  is retrieved for that Spec.

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