Add handling of parent being absent from cache and parent being self

[stroncium]

Fixes #15

Now falls back to normal require(parent being import-fresh then) if actual parent is not in cache.

Also, handles parent being the same as target by changing parent to import-fresh too. (I'd suspect it is a really rare edge case, but memory leak from repeated import-freshing itself would be quite hard to debug.)

//NOTE additionally tested with eslint/eslint#12529

[sindresorhus]

Thanks for fixing :)

[mdjermanovic]

Thanks! :-)