Add no-undeclared-imports eslint rule

[joehan]

Description

Add no-undeclared-imports eslint rule - this means importing something not declared in package.json (like an indirect dependency) will be a linter error.

This also caught a few cases of us using types from indirect dependencies - this isn't a runtime error (since types get washed away during compile), but it is ugly, so I added those as devDeps.

[joehan]

Broke the build, will resend once i fix that.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 51.15%. Comparing base (49c5ac5) to head (60ceb90).
Report is 12 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8017      +/-   ##
==========================================
- Coverage   51.19%   51.15%   -0.05%     
==========================================
  Files         423      423              
  Lines       29544    29577      +33     
  Branches     6031     6040       +9     
==========================================
+ Hits        15125    15130       +5     
- Misses      13059    13087      +28     
  Partials     1360     1360              

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