Extracted from #45189

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.