Ensure that any __non_webpack_require__ is retained in subsequent builds

[guybedford]

This is an interesting one, basically when an ncc build is again built, any instances of the following pattern:

const requireFunc = typeof __webpack_require__ === "function" ? __non_webpack_require__ : require;

will be converted into:

const requireFunc = false ? require : undefined;

by Webpack. And as a result the pattern breaks on the second build.

Fixed by injecting a phase before that avoids this double rewriting.

//cc @sokra this seems to be a breakdown in build transitivity in Webpack.

[codecov-io]

Codecov Report

Merging #271 into master will increase coverage by 0.16%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #271      +/-   ##
==========================================
+ Coverage   74.91%   75.08%   +0.16%     
==========================================
  Files          17       17              
  Lines         909      915       +6     
==========================================
+ Hits          681      687       +6     
  Misses        228      228

Impacted Files	Coverage Δ
src/loaders/relocate-loader.js	97.36% <100%> (+0.06%)	⬆️

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