Move externals plugin into configs

[mwistrand]

Type: bug

The following has been addressed in the PR:

• There is a related issue
• All code has been formatted with prettier
• Unit or Functional tests are included in the PR

Description:

Move the ExternalLoaderPlugin into the configs since it must be registered after the HtmlWebpackPlugin in order to function correctly.

Resolves dojo/webpack-contrib#119

[codecov]

Codecov Report

Merging #234 into master will decrease coverage by 0.36%.
The diff coverage is 25%.

@@            Coverage Diff            @@
##           master    #234      +/-   ##
=========================================
- Coverage   52.97%   52.6%   -0.37%     
=========================================
  Files          10      10              
  Lines         606     614       +8     
  Branches      148     154       +6     
=========================================
+ Hits          321     323       +2     
- Misses        285     291       +6

Impacted Files	Coverage Δ
src/base.config.ts	22.85% <ø> (-0.05%)	⬇️
src/dev.config.ts	21.42% <25%> (+0.27%)	⬆️
src/dist.config.ts	31.74% <25%> (-0.46%)	⬇️
src/base.test.config.ts	13.79% <25%> (+1.79%)	⬆️

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