bug#56754: 28.1; Please move general stuff from `ffap.el' to `files.el'

[Drew Adams]

File `ffap.el' includes utility functions that have nothing particular
to do with ffap.  Please consider moving at least some such functions to
`files.el', so that 3rd-party code that want so use them doesn't need to
depend on `ffap.el'.

One example of such a function: `ffap-all-subdirs' (and its internal
helper function `ffap-all-subdirs-loop').

Yes, a library that doesn't want to require `ffap.el' could always get
the same effect using more general file utilities - in this case
`directory-files-recursively' (matching a vacuous regexp and providing a
predicate).  But something like `ffap-all-subdirs' is convenient for
doing just what it does.

`ffap.el' seems to have evolved general stuff in isolation - stuff it
uses but which is by no means specific to find-file-at-point.  Maybe
start by moving a few such functions, even just `ffap-all-subdirs', if
you don't want to go to a lot of trouble immediately.

In GNU Emacs 28.1 (build 2, x86_64-w64-mingw32)
 of 2022-04-21 built on AVALON
Windowing system distributor 'Microsoft Corp.', version 10.0.19044
System Description: Microsoft Windows 10 Pro (v10.0.2009.19044.1826)