fix(plugin-webpack): properly reference index.js in production

[shimaore]

When building an executable using electron-forge make, setting publicPath as / leads (for example) to index.js being referenced as file:///main_window/index.js, which cannot be found. (The URL points to /main_window/index.js, which is incorrect. It should be ../main_window/index.js.)

When it is unset, the build runs properly and provides the correct URL. (This still works with electron-forge start as well with manual reloading.)

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• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

Removed the (apparently unnecessary) reference to publicPath.
Removed the reference to publicPath when building for production.

[malept]

Hmmm. This change didn't actually make the reference ../main_window/index.js, it's still /main_window/index.js 😢