A bit further on dpkg aliasing support

Hi,

I've had a bit of time again, and got it to a state where I can detect 
and reject packages that try to unpack conflicting files. There is also 
a WIP tip on the branch that implements rudimentary Replaces handling, 
but that needs cleanup, because it copies the "does A replace B" code 
from unpacking instead of moving that to a library or similar.

What's also not great is how removing files from other packages works -- 
I've mostly stolen that from regular unpacking, but it feels 
insufficiently atomic to me (but so does the normal unpacking path).

Current state: https://salsa.debian.org/sjr/dpkg/-/tree/wip/alias

There are quite a few functions that take a pointer to a pkginfo and a 
pkgbin, and there seems to be an implicit understanding that the pkgbin 
is either the "installed" or the "available" member of the pkginfo. Does 
it make sense to replace the second pointer by a two-value enum?

Also, while the entire thing is not yet ready for merging, getting 
feedback on some commits might be useful still to see if I'm going the 
right way at all here.

    Simon