compose: Add "remove" verb

I'd been resisting this for a long time - I really wanted the tree to
be a reflection of the packages; and not go down the path of
"forking".

The lorax model has shown that while you can definitely get a large
space reduction that way, you're going to be perpetually chasing
changes in the packages.  No matter how good your templates are.

Furthermore, lorax is just to generate the installer - it's an OS that
runs a single app.  Whereas here we're generating the target runtime
system; we can't add bugs.

Nevertheless, reality is that sometimes it's just too hard to change
the input package set - there's a risk of breaking things.  Namely,
we're introducing a new update system here, but obviously there's a
previous one: yum.  Upstream packages keep growing a dependency on it.

So in the meantime while we're iterating on this, let's support:

  "remove": ["usr/bin/yum"],

The code is generic beyond yum for obvious reasons, but I don't
think we should use it for a lot more than that.