API routes are broken in development for serverless target

[kevva]

When creating API routes introduced in #7296 using the serverless target, they break in development because of this line where it tries to resolve the files in the serverless folder which doesn't exist in development. The solution would be to add a check for this.renderOpts.dev as we do here.

You can reproduce this by doing the following:

1. Create a API route in pages/api
2. Run now dev
3. Try accessing the route
4. There should be an error

[lfades]

@huv1k Will your PR fix this ?

[huv1k]

@lfades yes :)