Publish package sources along with transpiled files

[Hypnosphi]

Issue: #2602

In #2589, I introduced jsnext:main fields so that eslint can resolve named and default exports when importing one package from another. They point to files in src directories that aren't published currently.

[codecov]

Codecov Report

Merging #2604 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2604   +/-   ##
=======================================
  Coverage   32.61%   32.61%           
=======================================
  Files         397      397           
  Lines        8855     8855           
  Branches      951      937   -14     
=======================================
  Hits         2888     2888           
- Misses       5317     5327   +10     
+ Partials      650      640   -10

Impacted Files	Coverage Δ
app/react/src/server/config/babel.js	0% <0%> (-100%)	⬇️
app/react/src/server/utils.js	0% <0%> (-53.58%)	⬇️
lib/ui/src/libs/key_events.js	40% <0%> (ø)	⬆️
addons/events/src/components/Event.js	38.55% <0%> (ø)	⬆️
addons/a11y/src/components/Report/Info.js	0% <0%> (ø)	⬆️
app/react-native/src/preview/story_kind.js	0% <0%> (ø)	⬆️
addons/info/src/components/types/Enum.js	0% <0%> (ø)	⬆️
addons/a11y/src/components/Report/index.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/api/configs/init_api.js	40.42% <0%> (ø)	⬆️
.../src/modules/ui/components/stories_panel/header.js	29.62% <0%> (ø)	⬆️
... and 43 more

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[igor-dv]

I think we don't want to publish src files...

[marcin-mazurek]

@igor-dv If you don't publish the src folder, then it doesn't make sense to set the esnext:main to src :)

[Hypnosphi]

I think we don't want to publish src files

What's exactly the problem with it? I find it very convenient to be able to view external packages' sources directly in node_modules directory

[tmeasday]

I don't feel very expert on this but I've seen it done before and it feels OK to me.

[tmeasday]

Happy to approve this

[ndelangen]

This would allow smart bundlers to tree-shake, correct?

LGTM

[Hypnosphi]

This would allow smart bundlers to tree-shake

Well we can actually to that ourselves, by tweaking our default webpack config =)
And I believe we're only ones who bundle our packages (so it raises the question, why even transpile before publishing)