Should not be merged before https://github.com/medusajs/medusa/pull/10895
**What**
- Introduces a new `plugin` command to `admin-bundler`, currently not used anywhere but will be called from `medusa build:plugin`
- Discovers plugins with extensions and add passes the to `admin-vite-plugin`.
- Updates `admin-vite-plugin` so its able to read built admin extensions.
Resolves CMRC-830, CMRC-839
**What**
- #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.
