solver: use a new heuristic (#46548)

This PR introduces a new heuristic for the solver, which behaves better when
compilers are treated as nodes. Apparently, it performs better also on `develop`,
where compilers are still node attributes.

The new heuristic:
- Sets an initial priority for guessing a few attributes. The order is "nodes" (300), 
  "dependencies" (150), "virtual dependencies" (60), "version" and "variants" (30), and
  "targets" and "compilers" (1). This initial priority decays over time during the solve, and
  falls back to the defaults.

- By default, it considers most guessed facts as "false". For instance, by default a node
  doesn't exist in the optimal answer set, or a version is not picked as a node version etc.

- There are certain conditions that override the default heuristic using the _priority_ of
  a rule, which previously we didn't use. For instance, by default we guess that a
  `attr("variant", Node, Variant, Value)` is false, but if we know that the node is already
  in the answer set, and the value is the default one, then we guess it is true.

Signed-off-by: Massimiliano Culpo <massimiliano.culpo@gmail.com>

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