concretizer: refactor to support multiple solver backends

There are now three parts:

- `SpackSolverSetup`
  - Spack-specific logic for generating constraints. Calls methods on
    `AspTextGenerator` to set up the solver with a Spack problem. This
    shouln't change much from solver backend to solver backend.

- ClingoDriver
  - The solver driver provides methods for SolverSetup to generates an ASP
    program, send it to `clingo` (run as an external tool), and parse the
    output into function tuples suitable for `SpecBuilder`.
  - The interface is generic and should not have to change much for a
    driver for, say, the Clingo Python interface.

- SpecBuilder
  - Builds Spack specs from function tuples parsed by the solver driver.

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