ASP-based solver: decouple setup phase from `clingo.backend` (#41952)

Currently, the `SpackSolverSetup` and the `PyclingoDriver` are more coupled than necessary:
1. The driver object needs a setup object to be injected during a solve, 
2. And the setup object will get a reference back to the driver

This design is necessary because we use the low-level `clingo.backend` interface to setup our problem. This interface though is meant to bypass the grounder and add symbols directly in the grounded table, which is a feature we don't currently use.

The PR simplifies the encoding by having the setup object returning the problem-specific facts / rules as a list of strings, and the driver ingesting them using the [clingo.Control.add](https://potassco.org/clingo/python-api/5.6/clingo/control.html#clingo.control.Control.add) method. This removes any use of the low level interface.

Using this encoding makes it easy to hash the output of the setup phase, since it is returned as a string.

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