fix(admin-vite-plugin): Normalize file paths and add tests

[kasperkristensen]

What

• feat(dashboard,admin-vite-plugin,admin-bundler,admin-sdk): Rework admin extensions and introduce custom fields API #9338 had a regression which caused the import path in some virtual modules to be invalid on Windows.
• This PR fixes the issue so we now again create the correct import paths, and adds tests to prevent this from slipping in again.

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Latest commit: 15e0847

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