Move more directories under pelican/

[jpmckinney]

• Move check-related directories to pelican/ (do this last, so it's easier to blame while doing the other Cleanup issues)
  • contracting_process
  • dataset
  • time_variance
• Rename files as needed for consistency and clarity
  • Checks should retain the same names to avoid rewriting large parts of pelican-frontend, and currently the file paths of check files match their string names.
  • We'll keep "resource_level" and "time_variance", since the SQL tables and phase enum have the same names.
  • We'll keep "contracting_process" as it's the same as the phase enum.
  • That said, we had already renamed the workers (data_item, time_based), since they form the user-facing API. Also, the docs use "compiled release-level" and "time-based" in English. (Noted in docs homepage.)

Should finish #20 first, as I anticipate that testing processor.py, examples.py, report.py, etc. will require some archaeology.

[jpmckinney]

Even now, I occasionally blame code to see if there's a reason for a methodological choice, so I'll leave it as-is.