Add solver config to control default the solver and displayed output

[dcookspi]

This adds solver configuration and options to set the default solver and default solver to display solver output from. This will let sites configure the solver/s they want, and let individual commands override that when required.

These changes add:

• --solver-to-show <SOLVER> argument to commands with Solver cli flags
• solver_to_run to the Solver section of the spk config file
• solver_to_show to the Solver section of the spk config file
• environment variable defaults to --solver-to-run and --solver-to-show. They use SPK_SOLVER__SOLVER_TO_RUN and SPK_SOLVER__SOLVER_TO_SHOW respectively to line up with the spk config file
• constants for the names used on the command line for the various solvers

Examples:

# Run all (both) the solvers and show the cli solver's output - this is the built-in default
spk explain python --solver-to-run all --solver-to-show cli

# Run all (both) the solvers and show the impossible checks solver's output
spk explain python --solver-to-run all --solver-to-show checks

# Run only the impossible checks solver and show its output
spk explain python --solver-to-run checks 

# Run all (both) the solvers and show the impossible checks solver's output
env SPK_SOLVER__SOLVER_TO_SHOW=checks spk explain spwidgets 

# Run only the cli solver and show its output
env SPK_SOLVER__SOLVER_TO_RUN=cli spk explain spwidgets