Support nested mod folders

[stupxd]

Seems to be the way things are heading /shrug

[flakywanderer]

This is something I meant to say earlier but forgot until now

My idea for the syntax for directory inclusions was something more like an include patch, like this:

[[patches.include]]
files = [
    "foo/bar.toml",
    "foo/baz.toml"
]
directories = [
    "xyzzy",
    "zzyzx"
]

[ethangreen-dev]

I think making nested directories data-oriented is the right approach, but I don't like making it another "patch". The right way to go about this is to standardize some sort of "lovely manifest", which would contain this information instead.

The lovely manifest is the big reason why this PR has not yet been reviewed - quite simply because it doesn't exist yet. I'll give this the review and merge it deserves once we're at a point where we have a design finalized.