Best effort co-concretization (iterative algorithm) (#28941)

Currently, environments can either be concretized fully together or fully separately. This works well for users who create environments for interoperable software and can use `concretizer:unify:true`. It does not allow environments with conflicting software to be concretized for maximal interoperability.

The primary use-case for this is facilities providing system software. Facilities provide multiple MPI implementations, but all software built against a given MPI ought to be interoperable.

This PR adds a concretization option `concretizer:unify:when_possible`. When this option is used, Spack will concretize specs in the environment separately, but will optimize for minimal differences in overlapping packages.

* Add a level of indirection to root specs

This commit introduce the "literal" atom, which comes with
a few different "arities". The unary "literal" contains an
integer that id the ID of a spec literal. Other "literals"
contain information on the requests made by literal ID. For
instance zlib@1.2.11 generates the following facts:

literal(0,"root","zlib").
literal(0,"node","zlib").
literal(0,"node_version_satisfies","zlib","1.2.11").

This should help with solving large environments "together
where possible" since later literals can be now solved
together in batches.

* Add a mechanism to relax the number of literals being solved

* Modify spack solve to display the new criteria

Since the new criteria is above all the build criteria,
we need to modify the way we display the output.

Originally done by Greg in #27964 and cherry-picked
to this branch by the co-author of the commit.

Co-authored-by: Massimiliano Culpo <massimiliano.culpo@gmail.com>

* Inject reusable specs into the solve

Instead of coupling the PyclingoDriver() object with
spack.config, inject the concrete specs that can be
reused.

A method level function takes care of reading from
the store and the buildcache.

* spack solve: show output of multi-rounds

* add tests for best-effort coconcretization

* Enforce having at least a literal being solved

Co-authored-by: Greg Becker <becker33@llnl.gov>

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