fix: require postcss module via resolver

[manniL]

Can be useful when trying to run a nuxt app in another folder, eg. via

yarn nuxt ../my-app or yarn nuxt -c ../my-app/nuxt.config.js

Otherwise, PostCSS plugins will be taken from the current working dir which probably doesn't have the plugins (in the above case it's the root dir of the nuxtjs repo)

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)

• 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 #4737 into dev will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev    #4737      +/-   ##
==========================================
+ Coverage   90.14%   90.18%   +0.04%     
==========================================
  Files          67       67              
  Lines        2282     2282              
  Branches      565      565              
==========================================
+ Hits         2057     2058       +1     
+ Misses        202      201       -1     
  Partials       23       23

Impacted Files	Coverage Δ
packages/webpack/src/utils/postcss.js	94.11% <100%> (ø)	⬆️
packages/core/src/resolver.js	85.48% <0%> (+1.61%)	⬆️

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