Avoid file peeking if evgen is running

This adds a mechanism for checking if we're running evgen and avoiding input file peeking in that case. It is not the most elegant thing I've ever written, but it is functional. The ugliness is contained to a single place (DerivationFrameworkMaster) which is an action-on-import module that needs to be modified into a function call at some point to make all of this easier - but that's a bit tricky, so here we're just getting the job done.