Move Downloads.jl from deps to weakdeps

[cake-duke]

It's only used in tests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.27%. Comparing base (6079713) to head (1b0d67f).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #47   +/-   ##
=======================================
  Coverage   52.27%   52.27%           
=======================================
  Files          12       12           
  Lines         461      461           
=======================================
  Hits          241      241           
  Misses        220      220           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.