Lift per_file_input_config outside of loop to avoid a potentially expensive clone

[DenialAdams]

My input config had 118,000 file object - each with DataSource::Contents - so we were spending tons (n * n) of time cloning here when it's not necessary

By hoisting the clone out of the loop and mutating thereafter, we avoid the expensive cloning.

This reduced my index build time from just under 3 hours to 55 seconds