fix(eslint-plugin): [no-unused-vars] correct detection of unused vars in a declared module with `export =`

[bradzacher]

If a declare module has an export = in its body, then TS will only export that.
If it doesn't have an export =, then all things are implicitly exported.

This adds handling to correctly detect this case.

declare module 'foo' {
  type T = 1; // T is unused and not exported
  const x = 2;
  export = x;
}

declare module 'bar' {
  type T = 1;
  const x = 2;
  // T and x are both implicitly exported
}

[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 #2505 into master will decrease coverage by 0.00%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master    #2505      +/-   ##
==========================================
- Coverage   92.80%   92.79%   -0.01%     
==========================================
  Files         290      290              
  Lines        9436     9451      +15     
  Branches     2642     2647       +5     
==========================================
+ Hits         8757     8770      +13     
  Misses        323      323              
- Partials      356      358       +2     

Flag	Coverage Δ
#unittest	92.79% <88.88%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
...t-plugin/src/rules/adjacent-overload-signatures.ts	93.33% <ø> (ø)
packages/eslint-plugin/src/rules/indent.ts	90.47% <ø> (ø)
packages/eslint-plugin/src/rules/no-unused-vars.ts	92.85% <85.71%> (-1.20%)	⬇️
packages/scope-manager/src/scope/ScopeBase.ts	91.71% <100.00%> (+0.04%)	⬆️