fix: do not emit emit deprecation warning while call deprecation plugin itself

[meskill]

This PR contains a:

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

Motivation / Use-Case

Deprecation warning

DeprecationWarning: less.webpackLoaderContext is deprecated and will be removed in next major release. Instead use pluginManager.webpackLoaderContext (https://webpack.js.org/loaders/less-loader/#plugins)

will be shown independently by user settings by the less-loader itself

This pr changes the way the deprecation warning plugin checks for already added webpackLoaderContext in lessProcessor

Breaking Changes

Additional Info

[codecov]

Codecov Report

Merging #414 (1a82eb6) into master (a48a770) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #414   +/-   ##
=======================================
  Coverage   96.87%   96.87%           
=======================================
  Files           4        4           
  Lines         128      128           
  Branches       32       32           
=======================================
  Hits          124      124           
  Misses          4        4           

Impacted Files	Coverage Δ
src/utils.js	97.59% <100.00%> (ø)

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[alexander-akait]

Weird, it should not output unnecessary warning, can you provide example how we can reproduce?

[alexander-akait]

Do not change this

[meskill]

Weird, it should not output unnecessary warning, can you provide example how we can reproduce?

I've tried to debug this part https://github.com/webpack-contrib/less-loader/blob/master/src/index.js#L34 with console logs

I do not know how it should work, but it seems like some files are handled in parallel, so webpackLoaderContext is still in less when new file get handled and not yet removed by this line https://github.com/webpack-contrib/less-loader/blob/master/src/index.js#L47

Logs in my project:

___init___
___render
__end
___init___
___render
__end
___init___
___render
__end
___init___
___render
__end
___init___
___render
__end
___init___
___render
___init___
___render
(node:11826) DeprecationWarning: less.webpackLoaderContext is deprecated and will be removed in next major release. Instead use pluginManager.webpackLoaderContext (https://webpack.js.org/loaders/less-loader/#plugins)
(Use `node --trace-deprecation ...` to show where the warning was created)
__end
__end

And based on #413 it is common issue