bug#38011: 27.0.50; [PATCH] WIP on allowing Gnus backends to return header data directly

[Eric Abrahamsen <eric@ericabrahamsen.net>]

I knew this seemed too simple...

Andy Cohen pointed out that `gnus-get-newsgroup-headers{,-xover}' were
doing more work (running hooks, and the alter function) that needed to
also be done if headers were returned directly. There's a bunch of other
overlap between those two functions (and between
`gnus-get-newsgroup-headers' and `nnheader-parse-naked-head') that Andy
is working on refactoring (I hope).

In the meantime there are several other places in the code that use
`gnus-retrieve-headers', and will need to be updated to handle the
possibility that headers have been returned directly. What I'd really
like to do, as much as possible, is to switch to `gnus-fetch-headers',
so we get our headers back and don't need to know how that happened.
This will also avoid ugliness like `nnvirtual-convert-headers'.

I think I can probably figure out how to do this in most spots.
gnus-agent and gnus-cache are particularly gnarly, though: what does
-braid-nov/-braid-heads actually do?

Eric