Fix `require('next')` when this plugin is pre-installed about next-runtime

[ehmicky]

First part done in #56

[ehmicky]

This is more complicated than expected. next-on-netlify requires @sls-next/lambda-at-edge which also requires next:

https://github.com/serverless-nextjs/serverless-next.js/blob/d32ee16feb738becc39a651af6b8aaa6c3e25658/packages/libs/lambda-at-edge/src/build.ts#L25

Therefore, I would suggest the following:

• Instead of the hack suggested above, add next@^9.5.3 as a production dependency of @netlify/plugin-nextjs
• This will make it harder for us to support Next.js 10, since this means @netlify/plugin-nextjs and next-on-netlify will use Next.js 9 even if the site is using Next.js 10. To fix this will require fixing how Build plugins are preinstalled (issue).

[ehmicky]

Done at #57