fix(eslint-plugin): [explicit-module-boundary-types] handle bodyless arrow functions with explicit return types that return functions

[bradzacher]

Fixes #2161

[typescript-eslint]

Thanks for the PR, @bradzacher!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

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[codecov]

Codecov Report

Merging #2169 into master will decrease coverage by 0.01%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##           master    #2169      +/-   ##
==========================================
- Coverage   93.47%   93.45%   -0.02%     
==========================================
  Files         171      171              
  Lines        7813     7810       -3     
  Branches     2229     2228       -1     
==========================================
- Hits         7303     7299       -4     
  Misses        244      244              
- Partials      266      267       +1     

Flag	Coverage Δ
#unittest	93.45% <40.00%> (-0.02%)	⬇️

Impacted Files	Coverage Δ
...plugin/src/rules/explicit-module-boundary-types.ts	89.14% <0.00%> (-0.09%)	⬇️
.../eslint-plugin/src/util/explicitReturnTypeUtils.ts	97.67% <50.00%> (-1.19%)	⬇️