refactor: remove Suggests dependence on covidcast

[dshemetov]

Checklist

Please:

• Make sure this PR is against "dev", not "main" (unless this is a release
  PR).
• Request a review from one of the current main reviewers:
  brookslogan, nmdefries.
• Makes sure to bump the version number in DESCRIPTION. Always increment
  the patch version number (the third number), unless you are making a
  release PR from dev to main, in which case increment the minor version
  number (the second number).
• Describe changes made in NEWS.md, making sure breaking changes
  (backwards-incompatible changes to the documented interface) are noted.
  Collect the changes under the next release number (e.g. if you are on
  1.7.2, then write your changes under the 1.8 heading).
• See DEVELOPMENT.md for more information on the development
  process.

Change explanations for reviewer

• just download the files, don't import covidcast
• covidcast depends on sf which is tough to install (it just refuses to compile on my machine) and my renv keeps trying to install ALL dependencies

Magic GitHub syntax to mark associated Issue(s) as resolved when this is merged into the default branch

• Resolves #{issue number}

[brookslogan]

I think we can do this more simply with a small addition to #520. Reviewing #520 now and going to try out.

[dshemetov]

Makes sense. Should probably be solved upstream in this case cmu-delphi/epidatasets#7

[brookslogan]

state_census at least is already there... but I guess county_census is not. Unfortunate, would require upstream work I guess. Hoped it was a little bit simpler.

[dshemetov]

It's basically a one line change to the epidatasets scripts, it's not bad

[brookslogan]

Think this is good to go. Going to force-push to try to make some checks happen.