Revert 37b0690fb82846d01e28dc26f0ad53fa5f2794cf.

This feature causes more problems than it solves.
Require _our_ resource_override.

Basically, when anyone else made a 'resource_override' generator,
it'd only require once. Because that's how require works. So let's make
sure that we require ours and nobody else's.

Fixes #251.
Sigh.

When I did 9551a974642d5b3e99068290a90e67bd27a41b0d, I
forgot that some new code may use it.