Ensure require.main === module behaviour

[guybedford]

Fixes #224

[codecov-io]

Codecov Report

Merging #229 into master will increase coverage by 0.17%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #229      +/-   ##
==========================================
+ Coverage      70%   70.17%   +0.17%     
==========================================
  Files          13       13              
  Lines         670      674       +4     
==========================================
+ Hits          469      473       +4     
  Misses        201      201

Impacted Files	Coverage Δ
src/loaders/relocate-loader.js	93% <100%> (+0.08%)	⬆️

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