Add subdirectory support to package scanning APIs

[mmarchetti]

Intent

This PR adds the dir parameter to the package scanning APIs: POST /api/packages/r/scan and POST /api/packages/python/scan.

Part of #1851

Type of Change

• • [ ] Bug Fix
• • [x] New Feature
• • [ ] Breaking Change
• • [ ] Documentation
• • [ ] Refactor
• • [ ] Tooling

Automated Tests

Added unit tests for the new parameter.

Directions for Reviewers

$(just executable-path) ui -vv --listen=localhost:9001 &
curl -s localhost:9001/api/packages/python/scan?dir=test/sample-content/rmd-static-1 -XPOST
curl -s localhost:9001/api/packages/r/scan?dir=test/sample-content/rmd-static-1 -XPOST

etc.

requirements.txt/renv.lock should be created in the specified subdirectory, and only contain package dependencies discoverable from the code in that subdirectory.