Update asset relocator loader

[guybedford]

This includes fixes for #273 and #280.

The unit tests for these are located in the commits there:

• vercel/webpack-asset-relocator-loader@590d2ab
• vercel/webpack-asset-relocator-loader@6122f38
• As well as the asset permissions fix in vercel/webpack-asset-relocator-loader@85bfce8.

[codecov-io]

Codecov Report

Merging #282 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #282   +/-   ##
=======================================
  Coverage   66.57%   66.57%           
=======================================
  Files          11       11           
  Lines         350      350           
=======================================
  Hits          233      233           
  Misses        117      117

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