Fix import path for Remix 1.6.2+

[haines]

Remix 1.6.2 changed the package layout, which breaks an import and causes typechecking errors:

node_modules/remix-flat-routes/dist/index.d.ts(1,57): error TS2307: Cannot find module '@remix-run/dev/config/routes' or its corresponding type declarations.

[kiliman]

Ah, I see. This was used by the tests. Since actual usage will pass in defineRoutes provided by Remix, that's why it didn't break in general.

I should probably just create a dummy defineRoutes for tests instead of doing a deep import (which may break again in the future, since that's technically an implementation detail).