Fix validation of sites without Next.js

[ehmicky]

When a site does not have Next.js, the following error message should be shown, so it is clear to users what should be fixed:

https://github.com/netlify/netlify-plugin-nextjs/blob/430569c6421f4447228ada44fe5821707f796d98/helpers/validateNextUsage.js#L9-L13

This requires importing next dynamically. This used to rely on next-on-netlify being dynamically imported. With next-on-netlify being merged, this stopped working and the following is shown instead (marked as a plugin bug in the build logs):

This should be fixed by importing next dynamically.

[ehmicky]

Also, if a site is using static exports (and does not have next), it will now fail.

[ehmicky]

Partially done at #123

[ehmicky]

Fixed by #124.