fix(react-scripts): use react-scripts' own postcss dependencies

[await-ovo]

Fixes#12177

When using npm 8, in some cases there are no postcss-flexbugs-fixes in the node_modules directory, instead they appear in node_modules/react-scripts/node_modules:

ls -l node_modules/react-scripts/node_modules | grep postcss

Therefore, we should use react-scripts' own postcss plugin dependencies, as well as postcss implementation.

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[merceyz]

Should be fixed in #11751 already

[cattermo]

This is not fixed in v5.0.1. I get error Loading PostCSS "postcss-normalize" plugin failed: Cannot find module 'postcss-normalize' if I try to use css module file from node_modules.