fix: improve contenthash generation

[alexander-akait]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

fixes #454

Breaking Changes

No

Additional Info

/cc @sokra

Do we need more logic for contenthash generation as in https://github.com/webpack/webpack/blob/master/lib/javascript/JavascriptModulesPlugin.js

[codecov]

Codecov Report

Merging #482 into master will decrease coverage by 0.12%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master     #482      +/-   ##
==========================================
- Coverage   88.49%   88.37%   -0.13%     
==========================================
  Files           5        5              
  Lines         426      430       +4     
  Branches       94       96       +2     
==========================================
+ Hits          377      380       +3     
- Misses         47       48       +1     
  Partials        2        2              

Impacted Files	Coverage Δ
src/index.js	87.61% <80.00%> (-0.24%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update aa22e44...e1c2a44. Read the comment docs.

[sokra]

These two are not part of the content of the css file

[alexander-akait]

@sokra without it, difference async chunks have same contenthash, but difference content, here example #454

[adriantran]

@sokra @evilebottnawi any update on this?

[yingzhe]

As far as I can tell, CSS modules don't have id property set so the hash is going to be the same. Would it be better to add hash.update(m.index2) (since index2 is used to sort the modules before rendering content asset) below after m.updateHash(hash) ?