Fix SSR external resolution with resolve.alias

[virtualzx-nad]

Original issue from vitejs/vite vitejs#19461

Currently, there's an issue where SSR external doesn't work correctly with resolve.alias. This needs to be fixed to ensure proper module resolution during SSR builds.

Steps to reproduce:

1. Set up a Vite project with SSR
2. Configure resolve.alias for a module
3. Mark the module as external for SSR
4. The module is not properly externalized

Required changes:

1. Fix the module resolution logic for SSR externals
2. Handle alias resolution properly
3. Add test cases
4. Update documentation if needed

This is a medium complexity issue that requires understanding of Vite's module resolution system.