ASP-based solver: allow to reuse installed externals (#31558)

fixes #31484

Before this change if anything was matching an external
condition, it was considered "external" and thus something
to be "built".

This was happening in particular to external packages
that were re-read from the DB, which then couldn't be
reused, causing the problems shown in #31484.

This PR fixes the issue by excluding specs with a
"hash" from being considered "external"

* Test that users have a way to select a virtual

This ought to be solved by extending the "require"
attribute to virtual packages, so that one can:
```yaml
mpi:
  require: 'multi-provider-mpi'
```

* Prevent conflicts to be enforced on specs that can be reused.

* Rename the "external_only" fact to "buildable_false", to better reflect its origin

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