Restore prior resolver default behavior but maintain direct code path for testing resolver.

[matteius]

The issue

Fixes #5803 by restoring the default behavior to run the resolver in the project python sub-process.

Maintain the option to invoke resolver directly (useful for debugging).

The checklist

• Associated issue
• A news fragment in the news/ directory to describe this fix with the extension .bugfix.rst, .feature.rst, .behavior.rst, .doc.rst. .vendor.rst. or .trivial.rst (this will appear in the release changelog). Use semantic line breaks and name the file after the issue number or the PR #.