The crucial change in this commit is the change to PackageFixedDeps to return a
ComponentDeps structure, rather than a flat list of dependencies, as long with
corresponding changes in ConfiguredPackage and ReadyPackage to accomodate this.

We don't actually take _advantage_ of these more fine-grained dependencies yet;
any use of

    depends

is now a use of

   CD.flatDeps . depends

but we will :)

Note that I have not updated the top-down solver, so in the output of the
top-down solver we cheat and pretend that all dependencies are library
dependencies.