--from-file vs pathlists - can we unify the way they work?

[ctb]

We have a few different issues around adding --from-file to new commands, e.g. #1631.

As I was implementing #1657 for a different reason, though, I realized that pathlists and --from-file seem completely redundant. If true, it's kind of embarrassing that I didn't realize it before :).

However, even if they end up doing the same thing, there is a big difference in the user experience for these two. --from-file adds all the lines from the specified file onto the argument list, while pathlists are loaded as a single MultiIndex database containing all of the different indexes etc.

I'm wondering if there's a way to modify _load_database to provide the same UX; maybe we could modify the loading functions to yield multiple databases, rather than just one, and have pathlist load yield multiple dbs?

[ctb]

related: #1878 and #1877

[ctb]

consider removing pathlists altogether per #1414 (comment)