Changing the -package dependency resolution algorithm

[Edward Z. Yang]

> We need to rethink the shadowing behaviour.  It is designed to handle 
> the case where we have the same PackageId (name + version) in two 
> different DBs (e.g. global and local).  Shadowing takes the topmost one 
> of these (e.g. local, or rightmost -package-db flag).  We can relax this 
> requirement so long as the InstalledPackageIds are different, but what 
> if the InstalledPackageIds are the same?  Right now that's OK, because 
> identical InstalledPackageIds implies identical ABIs, but if we change 
> that so that InstalledPackageId is derived from the source and not the 
> ABI, we would not be able to assume that two identical 
> InstalledPackageIds are compatible.

I talked to Duncan about this, and he's asserted that under a Nix-like
model, equal InstalledPackageIds really would imply identical ABIs.
I think this would be pretty hard to achieve reliably (and if we get
it wrong, segfault); Simon, perhaps you would know more about this.

Duncan (and shout if I understand wrong) is also keen on abolishing the
shadowing algorithm completely when we have package environments and
making a package environment mandatory (defaulting to a global package
environment when nothing available.)  The reason for this is that in a
Nix model, we mostly abolish package database stacks and have a single
global package database, which all packages are chucked into.  In this
case, the current shadowing really has no idea how to pick between two
packages which shadow each other in the same database.

Edward