fix: source map generation

[alexander-akait]

This PR contains a:

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

Motivation / Use-Case

fixes #652

partial #886

Breaking Changes

No

Additional Info

Avoid prefix, because postcss generate invalid source map, also normalize path in source map from other loader (node-sass generate forward slash in source map, but it is invalid because source map using URLs, paths are invalid)

[codecov]

Codecov Report

Merging #901 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #901      +/-   ##
==========================================
- Coverage   99.25%   99.25%   -0.01%     
==========================================
  Files          10       10              
  Lines         402      401       -1     
  Branches      118      119       +1     
==========================================
- Hits          399      398       -1     
  Misses          3        3

Impacted Files	Coverage Δ
src/index.js	100% <100%> (ø)	⬆️

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