fix(eslint-plugin): [no-unused-vars] better handling for declared modules

[bradzacher]

Fixes #2523

• declare global should never be marked as unused.
• namespaces within declared namespaces all ambiently export their children

[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!

---

🙏 Please, if you or your company is finding typescript-eslint valuable, help us sustain the project by sponsoring it transparently on https://opencollective.com/typescript-eslint. As a thank you, your profile/company logo will be added to our main README which receives thousands of unique visitors per day.

[codecov]

Codecov Report

Merging #2553 into master will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2553   +/-   ##
=======================================
  Coverage   92.81%   92.81%           
=======================================
  Files         290      290           
  Lines        9453     9456    +3     
  Branches     2647     2648    +1     
=======================================
+ Hits         8774     8777    +3     
  Misses        322      322           
  Partials      357      357           

Flag	Coverage Δ
#unittest	92.81% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
packages/eslint-plugin/src/rules/no-unused-vars.ts	93.06% <ø> (+0.21%)	⬆️