Browsing a set of dir of files in a single buffer

[Stefan Monnier]

Org (and `outline-mode` before it) provide a hierarchical structuring
similar to that of files and directories.  I like that better than
directories, because I usually find it easier to navigate a big file
than several small files in a directory.

But I don't always get to choose, and sometimes I'm stuck browsing
several small files in a directory hierarchy.

Does anyone know a nice Emacs package that lets me view that whole
hierarchy in a single buffer, as if it were something like a big
`outline-mode` file?

It'd be a bit like dired-mode but where you can see (and hopefully edit)
the files directly in the dired buffer simply by "folding/unfolding" them.


        Stefan