windows: Make windows/main.c independent from unix/main.c

Up to know changes unix/main.c were able to break the windows build because windows/main.c simply included unix/main.c.

Now windows/main.c stands on it's own and won't break if unix/main.c changes.

Also, import paths have been fixed on Windows ([line 149](https://github.com/msiemens/micropython/commit/7e0dee0cbf9807261cea39321030887504172b80#diff-2b6f82badc178a105588853f7bd8b13fR149))