Move builds to core

[igor-dv]

Issue: #2636

What I did

1. Moved index.js to core
2. Moved build.js to core
3. Fixed polymer leftovers - (UglifyJsPlugin and getConfigDir)
4. Moved WatchMissingNodeModulesPlugin to core

[codecov]

Codecov Report

Merging #2937 into master will increase coverage by 1.38%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2937      +/-   ##
==========================================
+ Coverage   35.99%   37.38%   +1.38%     
==========================================
  Files         429      426       -3     
  Lines        9503     9150     -353     
  Branches      976      869     -107     
==========================================
  Hits         3421     3421              
+ Misses       5438     5226     -212     
+ Partials      644      503     -141

Impacted Files	Coverage Δ
app/angular/src/server/utils.js	0% <ø> (ø)	⬆️
app/vue/src/server/config/webpack.config.js	0% <ø> (ø)	⬆️
app/react/src/server/utils.js	100% <ø> (+100%)	⬆️
app/angular/src/server/config/webpack.config.js	0% <ø> (ø)	⬆️
...src/server/config/WatchMissingNodeModulesPlugin.js	0% <ø> (ø)
...p/polymer/src/server/config/webpack.config.prod.js	0% <ø> (ø)	⬆️
app/polymer/src/server/utils.js	0% <ø> (-53.58%)	⬇️
lib/core/src/server/middleware.js	0% <ø> (ø)
app/vue/src/server/utils.js	0% <ø> (ø)	⬆️
app/polymer/src/server/config/webpack.config.js	0% <ø> (ø)	⬆️
... and 88 more

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[Hypnosphi]

WatchMissingNodeModulesPlugin is actually a part of react-dev-utils package which we already depend on, in all apps (I guess, react is in the package name only because it's a part of create-react-app project. Let's import it from there instead of replicating on our side

[igor-dv]

Makes sense to me

[Hypnosphi]

Ok, I'll make a PR

[Hypnosphi]

#3141