fix(webpack): Fix webpack loader when another loader used before

[madzhup]

Use proper source when using another loader before svgr/webpack

[codecov]

Codecov Report

Merging #139 into master will increase coverage by 0.07%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master     #139      +/-   ##
==========================================
+ Coverage   88.55%   88.63%   +0.07%     
==========================================
  Files          26       26              
  Lines         428      431       +3     
  Branches      113      119       +6     
==========================================
+ Hits          379      382       +3     
+ Misses         37       36       -1     
- Partials       12       13       +1

Impacted Files	Coverage Δ
packages/webpack/src/index.js	95.45% <87.5%> (+0.71%)	⬆️
packages/core/src/plugins/h2x.js	96.29% <0%> (ø)	⬆️

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[gregberge]

Thanks!