fix: get source code from sourceMapStore on write report

[billowz]

When the fileCoverage.inputSourceMap have different source content, should be get the source code from the "inputSourceMap" on writing report.

[codecov-io]

Codecov Report

Merging #72 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master     #72   +/-   ##
======================================
  Coverage    97.7%   97.7%           
======================================
  Files           2       2           
  Lines         131     131           
======================================
  Hits          128     128           
  Misses          3       3

Impacted Files	Coverage Δ
src/reporter.js	97.77% <100%> (ø)	⬆️

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[mattlewis92]

Nice one, thank you! 😄