fix(eslint-plugin): [no-unused-vars] fix false positives for duplicated names in namespaces

[mbpreble]

Fixes #2595

This attempts to make markDeclarationChildAsUsed somewhat more targeted.
In some cases it seems that context.getScope().upper is the correct scope where we should attempt to mark a variable as having been used, in order to avoid starting further down the tree and marking a variable in a child scope as used instead (context.markVariableAsUsed is an inside-out search, keying in on variable name only).

Tests have been added for the two cases where there appears to be a false positive (the ambient declaration of either a function or a namespace with a variable of the same name of the parent in scope)

[typescript-eslint]

Thanks for the PR, @mbpreble!

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 #2659 into master will increase coverage by 0.04%.
The diff coverage is 90.00%.

@@            Coverage Diff             @@
##           master    #2659      +/-   ##
==========================================
+ Coverage   92.80%   92.85%   +0.04%     
==========================================
  Files         293      294       +1     
  Lines        9604     9665      +61     
  Branches     2711     2712       +1     
==========================================
+ Hits         8913     8974      +61     
  Misses        326      326              
  Partials      365      365              

Flag	Coverage Δ
#unittest	92.85% <90.00%> (+0.04%)	⬆️

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	93.13% <90.00%> (+0.06%)	⬆️
packages/scope-manager/src/lib/index.ts	100.00% <0.00%> (ø)

[bradzacher]

LGTM - thanks for your contribution!