Treat roxygen as a dependency

[infotroph]

Description

Switches roxygen2 from manually installed on the depends image to automatically handled by generate_dependencies.R and pecan.depends.R.

Note that the Makefile continues to check for (and install as needed) a hard-coded version -- this seems prudent to keep as protection against accidental Roxygen updates on dev machines.

Motivation and Context

I am hoping this will make life easier in two ways:

• Faster builds on the Depends image: The existing Dockerfile installs roxygen2 7.2.3 from CRAN before calling the depends script, and I note that
  1. We don't expect Roxygen to be needed during dependency installation, and if it is it's OK to use any version that's available -- it doesn't have to be 7.2.3 specifically.
  2. Installing roxygen2 from cran.rstudio.com resulted in many of Roxygen's dependencies being compiled from source even though they are available precompiled from the Posit package manager. By moving Roxygen installation into the binary-enabled step, we should be able to save a fair amount of compiling.
• Simplify Roxygen version management. After this change, the approved version of Roxygen lives exactly one place (the Makefile) and all others will update from there. I think updating to a new version of roxygen should consist of:
  1. Find and replace all 7.2.3s in the Makefile to the version you want to use.
  2. Run make document, which will install the correct version of Roxygen2 and reroxygenize all the packages.
  3. Run ./scripts/generate_dependencies.R
  4. Inspect all changes, commit, push, confirm CI passes, merge.

Review Time Estimate

• [ ] Immediately
• [ ] Within one week
• [ ] When possible

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ ] My change requires a change to the documentation.
• [ ] My name is in the list of CITATION.cff
• [ ] I have updated the CHANGELOG.md.
• [ ] I have updated the documentation accordingly.
• [ ] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.