perf: use modules only if explicitly set

[alexander-akait]

What kind of change does this PR introduce?
bugfix

Did you add tests for your changes?

Already have.

If relevant, did you update the README?

After accept.

Summary

Now css-loader it is not loader for css, many project don't use css-modules for their project, but seems (https://github.com/webpack-contrib/css-loader/blob/master/lib/processCss.js#L152) they are use always. We spend a lot of time on postcss-modules-*, even if I set modules: false. I think this is completely wrong and many issue about performance probably related to this. Right name for this loader is css-modules-loader.

postcss-loader in v2 (i hope we can release it faster) wants to abandon css-loader, allow use only postcss-loader handle css and etc stuff (url, imports and etc). In fact, this is the real css-loader, It is simple, does not contain any logic.

What we can do?

1. Merge this PR before release css-loader (css-modules-loader) v1 and postcss-loader v2 (fix performance for project don't use css-modules).
2. Move all logic not related to css-modules (urls, improts and etc stuff) to postcss-loader.
3. Deprecated css-loader in favor css-modules-loader.
4. ...
5. PROFIT (better performance to project don't using css-modules, clear logic for postcss-loader and css-modules-loader, separation of issues - related to css-modules in css-modules-loader, related to css in postcss-loader)

Ref about postcss-loader v2: webpack-contrib/postcss-loader#177

Does this PR introduce a breaking change?

Should not.

Other information

Not required.

[codecov]

Codecov Report

Merging #497 into master will decrease coverage by 0.23%.
The diff coverage is 97.72%.

@@            Coverage Diff             @@
##           master     #497      +/-   ##
==========================================
- Coverage    98.3%   98.07%   -0.24%     
==========================================
  Files          10       10              
  Lines         354      363       +9     
  Branches       79       84       +5     
==========================================
+ Hits          348      356       +8     
- Misses          6        7       +1

Impacted Files	Coverage Δ
lib/localsLoader.js	100% <ø> (ø)	⬆️
lib/loader.js	100% <ø> (ø)	⬆️
lib/processCss.js	97.45% <97.72%> (-0.47%)	⬇️

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[michael-ciniawsky]

@evilebottnawi I take a deeper look later, but this is also related to the removal of

:local .className local(.className) {...}

then, which is the main reason to always use CSS Modules atm, but it's a bad practice anyways 😛

[michael-ciniawsky]

@evilebottnawi Agian this would be definitely a breaking change ? Should be because of :local(.className) {} not working anymore. In this case we need to close this PR and move on as discussed, since the next semver major of css-loader will definitely not contain CSS Modules anymore 😛

[alexander-akait]

@michael-ciniawsky agree, let's close this and do all in steps 😄