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

[ehmicky]

In production, we pre-install Build plugins in /opt/buildhome/.netlify-build-plugins. However, the repository is in /opt/build/repo. Therefore, trying to require a dependency of the site (also known as peer dependency) from a plugin does not work. This problem is described in details in this issue.

In the case of both this plugin and next-on-netlify, we are doing require('next'). We are doing this to allow users to choose their own Next.js version instead of forcing one (see #25). This currently fails in production due to the problem above.

The proper solution to this problem is detailed in this issue. However, this is not a quick fix, so we need an alternative in the meantime.

The only solution I can think of is the following: if we detect that the plugin is run in production, use require.resolve() to locate next. In practice this would mean lines like:

const { PHASE_PRODUCTION_BUILD } = require('next/constants')

would be changed to:

const requirePeerDependency = function(modulePath) {
  if (process.env.NETLIFY === 'true') {
    return require(require.resolve(modulePath, { paths: ['.'] } }))
  }

  return require(modulePath)
}

const { PHASE_PRODUCTION_BUILD } = requirePeerDependency('next/constants')

This is hacky. This would need to be done in next-on-netlify too.

What do you think?

[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