Plugin load if `next` is not available

[ehmicky]

next-on-netlify requires next as a peerDependency:

https://github.com/netlify/next-on-netlify/blob/7d1fd5e08969455035bf9918a6fb2d707984c4a1/package.json#L37-L39

This repository requires next-on-netlify as a production dependency:

https://github.com/netlify/netlify-plugin-nextjs/blob/ef4529016ed0f87ae89d5f312dc8f8f08d1ef7b6/package.json#L24-L29

If a site is using this plugin, but forgot to install next, this plugin will thrown when being require()'d, as opposed to when onPreBuild is run. This results in an error message which might be confusing:

Uncaught Error: Cannot find module 'next/constants'
Require stack:
- /home/ether/Desktop/node_modules/next-on-netlify/lib/helpers/getNextDistDir.js
- /home/ether/Desktop/node_modules/next-on-netlify/lib/config.js
- /home/ether/Desktop/node_modules/next-on-netlify/lib/steps/prepareFolders.js
- /home/ether/Desktop/node_modules/next-on-netlify/index.js
- /home/ether/Desktop/node_modules/@netlify/plugin-nextjs/index.js
- <repl>
    at Function.Module._resolveFilename (node:internal/modules/cjs/loader:900:15)
    at Function.Module._load (node:internal/modules/cjs/loader:745:27)
    at Module.require (node:internal/modules/cjs/loader:972:19)
    at require (node:internal/modules/cjs/helpers:88:18) {
  code: 'MODULE_NOT_FOUND',
  requireStack: [
    '/home/ether/Desktop/node_modules/next-on-netlify/lib/helpers/getNextDistDir.js',
    '/home/ether/Desktop/node_modules/next-on-netlify/lib/config.js',
    '/home/ether/Desktop/node_modules/next-on-netlify/lib/steps/prepareFolders.js',
    '/home/ether/Desktop/node_modules/next-on-netlify/index.js',
    '/home/ether/Desktop/node_modules/@netlify/plugin-nextjs/index.js',
    '<repl>'
  ]
}

To fix this:

• next and next-on-netlify should be require()'d only inside the if (isNextProject) { ... } block
• _isNextProject() should try to require('next'), and return false if that throws an error

We might want to also wonder: are we ok with this plugin requiring users to install Next.js as a dependency (as opposed to doing it for them)?

[lindsaylevine]

fair enough, happy to add this! but also would be very confused if a next app didnt have next installed, they wouldnt be able to develop their app locally unless they weirdly had next globally installed

[ehmicky]

This is reported on Bugsnag as well.

[ehmicky]

Related to #25

[ehmicky]

See also #25 (comment)

[ehmicky]

Fixed by #45