fix(webpack): chunkhash and contenthash not work with extractCSS

[clarkdo]

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

Fix #6439

extractCSS HMR is unnecessary to be enabled and it will generate timestamp in chunk source, so the chunkhash will be different in multiple builds of same file

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly. (PR: #)
• I have added tests to cover my changes (if not applicable, please state why)
• All new and existing tests are passing.

[codecov-io]

Codecov Report

Merging #6520 into dev will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev    #6520      +/-   ##
==========================================
+ Coverage   95.67%   95.67%   +<.01%     
==========================================
  Files          79       79              
  Lines        2706     2707       +1     
  Branches      698      698              
==========================================
+ Hits         2589     2590       +1     
  Misses        101      101              
  Partials       16       16

Flag	Coverage Δ
#e2e	100% <ø> (ø)	⬆️
#fixtures	50.6% <100%> (+0.01%)	⬆️
#unit	92.35% <0%> (-0.04%)	⬇️

Impacted Files	Coverage Δ
packages/webpack/src/utils/style-loader.js	94.28% <100%> (+0.16%)	⬆️

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