fix(plugin-webpack): validate that the correct entry point is used

[malept]

• I have read the contribution documentation for this project.
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• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

Addresses #2519

Basically, we can do this because afterCopy hooks are executed before the entry point validation in Electron Packager.

[codecov]

Codecov Report

Merging #2522 (c39429d) into master (0deb74b) will increase coverage by 2.42%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2522      +/-   ##
==========================================
+ Coverage   72.71%   75.13%   +2.42%     
==========================================
  Files          91       91              
  Lines        2349     2353       +4     
  Branches      429      430       +1     
==========================================
+ Hits         1708     1768      +60     
+ Misses        471      470       -1     
+ Partials      170      115      -55     

Impacted Files	Coverage Δ
packages/plugin/webpack/src/WebpackPlugin.ts	37.30% <100.00%> (+0.65%)	⬆️
packages/api/core/src/util/parse-archs.ts	83.33% <0.00%> (+3.33%)	⬆️
packages/api/core/src/api/init.ts	97.22% <0.00%> (+5.55%)	⬆️
packages/api/core/src/util/rebuild.ts	100.00% <0.00%> (+5.55%)	⬆️
packages/api/core/src/api/make.ts	88.77% <0.00%> (+8.16%)	⬆️
packages/api/core/src/api/install.ts	92.10% <0.00%> (+14.47%)	⬆️
packages/api/core/src/api/publish.ts	90.90% <0.00%> (+21.17%)	⬆️
packages/api/core/src/api/start.ts	87.65% <0.00%> (+22.22%)	⬆️

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