Remotes cleanup refactoring, code cleanup & bug fixing

[averissimo]

Overview of changes description

note: I've added some comments in the code below for additional context.

• desc_remotes_cleanup() refactoring just as we discussed
  • I couldn't remove Remotes while keeping functionality
• Adds tests
• Code reorganization
  • Moved solve_* functions to own file
  • Moved some functions around (especially to desc_util.R, for description-based functions)
  • local_description() moved to R/
    • Useful function to create better examples for developers
• when resolving packages: Replaces remote_github_ref to CRAN when resolving packages if that version (rule) exists on CRAN
  • For example: formatters (>= 0.5.1) isn't on CRAN, so it keeps the github_ref (instead of replacing with non-existent CRAN versions)
• Fix Bioconductor lack of published date bug (see comment below)
  • parse_ppm_url() fallback to lastest from PPM if snapshot date cannot be resolved
• Adds utility map_key_character to avoid repetitive vapply()

[pawelru]

This is awesome. Thank you! I have approved this. Let's merge it to the training branch and then the training to the main. I will let you decide when you want to pull the trigger.