entity_lookup should support file/image destination fields

Working on #2635622: Process plugin for importing/creating files by URL, I'm close to making file entity generation work without having a new process plugin extending entity_generate (leveraging the file_copy plugin I'll introduce in that patch)... The problem with using entity_generate directly is the entity_lookup

1. Assumes the destination field is an entity_reference (file and image fields are sufficiently entity_reference-like to be leveraged, I think), and
2. Assumes the entity being generated is bundleable (is that a word)?

So, I think generalizing entity_lookup will save us from having a specific file_generate process plugin, and be more flexible in general.