GitLab

Uses Data.Graph and annoyingly we also need to keep functions
around for mapping between Graph.Vertex <-> PackageIdentifier

For example it's not allowed for installed packages to depend
on configured packages.

Instead of just reporting that a plan is invalid, produce a
detailed list of reasons why it is invalid.
This should be useful for people debugging dependency resolvers.
Also rename the complete property to closed, since the property
is about the set being closed under the dependency relation.
Also re-use the PackageIndex functions for checking the validity
conditions rather than re-implementing the checks locally.

Renamed to AvailablePackage since that what it is really.
Now instead of just representing packages from a remote hackage repo
it includes an alternative for a local unpacked package. In future
we should add more alternatives, eg for other local packages (ie not
just the one that's unpacked in the current dir) and for remote
packages in source control like darcs, git etc.

Make it usable for callers that want to check
properties before constructing an InstallPlan.

This is joint work with Kolmodin.
An InstallPlan records what we want to install and records the outcome
of installing packages. It has a fairly strict internal invariant which
should guarantee that each package can be configured sucessfully.