fix: fix find-cache-dir missing dependency

[ehmicky]

Fixes #126.

We use the find-cache-dir dependency but do not include it in our package.json dependencies.

https://github.com/netlify/netlify-plugin-nextjs/blob/61b61f57d58ae095e3522c86013c75823911179b/src/lib/steps/prepareFolders.js#L3

https://github.com/netlify/netlify-plugin-nextjs/blob/61b61f57d58ae095e3522c86013c75823911179b/src/lib/helpers/handleFileTracking.js#L3

This creates the following bug in production:

Could not import plugin:
Cannot find module 'find-cache-dir'
Require stack:
- /opt/build/repo/.netlify/plugins/node_modules/@netlify/plugin-nextjs/src/lib/steps/prepareFolders.js
- /opt/build/repo/.netlify/plugins/node_modules/@netlify/plugin-nextjs/src/index.js
- /opt/build/repo/.netlify/plugins/node_modules/@netlify/plugin-nextjs/index.js
- /opt/buildhome/.netlify-build-nvm/versions/node/v12.16.3/lib/node_modules/@netlify/build/src/plugins/child/logic.js
- /opt/buildhome/.netlify-build-nvm/versions/node/v12.16.3/lib/node_modules/@netlify/build/src/plugins/child/load.js
- /opt/buildhome/.netlify-build-nvm/versions/node/v12.16.3/lib/node_modules/@netlify/build/src/plugins/child/main.js 
    internal/modules/cjs/loader.js:957:15 Function.Module._resolveFilename
    internal/modules/cjs/loader.js:840:27 Function.Module._load
    internal/modules/cjs/loader.js:1019:19 Module.require
    internal/modules/cjs/helpers.js:77:18 require
    /opt/build/repo/.netlify/plugins/node_modules/@netlify/plugin-nextjs/src/lib/steps/prepareFolders.js:3:22 Object.<anonymous>
    internal/modules/cjs/loader.js:1133:30 Module._compile
    internal/modules/cjs/loader.js:1153:10 Object.Module._extensions..js
    internal/modules/cjs/loader.js:977:32 Module.load
    internal/modules/cjs/loader.js:877:14 Function.Module._load
    internal/modules/cjs/loader.js:1019:19 Module.require

find-cache-dir is a dependency of next, so it turns out the module can still be required most of the times. However, there are cases where this is not the case, e.g. when next is not a site dependency (see #117).