refactor(tests): use devtools fixtures

[wpbonelli]

Update tests that load models from external repos to use devtools fixtures, with a bit of other refactoring. This was carved out of #1066 for a smaller changeset and more incremental refactor.

• Use fixtures from modflow-devtools to:
  • map target executable names to their paths (targets fixture)
  • parametrize tests with models loaded from external repos
    • the set of models tested is not changed — coupled GWT models are skipped, as before
    • support example selection by model name/package via --model/--package pytest CLI options
      • there is a package filtering bug in devtools, soon to be fixed in fix(get_packages): fix get_packages(), fix model filtering by package modflow-devtools#44
    • support --original-regression flag as a pytest CLI option/fixture (configured in new conftest.py file)
    • update DEVELOPER.md to reflect devtools convention that external model repos live side by side with modflow6, or optional REPOS_PATH environment variable to specify a different location
• Use devtools and flopy instead of pymake in build_exes.py and get_exes.py. Give both scripts an argparse CLI with optional --path argument to specify a different bin path (defaults to the existing location, bin in the project root).
• Remove pymake usage from framework/simulation and external model tests. Move some utility functions previously in pymake into common_regression.py. A followup PR could replace pymake usages in the rest of the tests.
• Use Github API token in CI step downloading exes (bypass rate limit)

[christianlangevin]

Looks great, @w-bonelli. Nice cleanup and nice documentation.