reuse concretization: allow externals from remote when locally configured (#35975)

This looks to me like the best compromise regarding externals in a
build cache. I wouldn't want `spack install` on my machine to install
specs that were marked external on another. At the same time there are
centers that control the target systems on which spack is used, and
would want to use external in buildcaches.

As a solution, reuse concretization will now consider those externals
used in buildcaches that match a locally configured external in
packages.yaml.

So for example person A installs and pushes specs with this config:

```yaml
packages:
  ncurses:
    externals:
    - spec: ncurses@6.0.12345 +feature
      prefix: /usr
```

and person B concretizes and installs using that buildcache with the
following config:

```yaml
packages:
  ncurses:
    externals:
    - spec: ncurses@6
    prefix: /usr
```

the spec will be reused (or rather, will be considered for reuse...)

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