fix(loader): only doing file I/O once during webpack loader.

[stephencranedesign]

Noticed that this webpack loader takes ~20/30s longer to build in my project then this loader: https://www.npmjs.com/package/swc-loader.

this change makes the load time the same in my project when I switch out that loader with this one.

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Latest commit: a721794d68d2a35e67972ddcda21371ed3dfe217

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