fix: support nested filenames in bundle for different OS

[ideffix]

In my project bundle sometimes contained nested path like somePath\index.css but normalizePath function is changing all back slashes to forward slashes. So if my bundle contains field somePath\index.css and normalizePath returned somePath/index.css I ended up with error:
[!] (plugin postcss) TypeError: Cannot read property 'modules' of undefined
This PR is fixing it by additionally checking bundle for non-normalizePath.

[codecov]

Codecov Report

Merging #393 (633dae1) into master (118253e) will increase coverage by 0.04%.
The diff coverage is 100.00%.

❗ Current head 633dae1 differs from pull request most recent head 23ad6ea. Consider uploading reports for the commit 23ad6ea to get more accurate results

@@            Coverage Diff             @@
##           master     #393      +/-   ##
==========================================
+ Coverage   92.94%   92.98%   +0.04%     
==========================================
  Files          10       10              
  Lines         326      328       +2     
  Branches      115      117       +2     
==========================================
+ Hits          303      305       +2     
  Misses         22       22              
  Partials        1        1              

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

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[ideffix]

@egoist any chance of having it merged?