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This required changing the PkgInfo type from an alias for GenericPackageDescription 
to a record with a Repo and a GenericPackageDescription.
ResolvedPackage and ResolvedDependency also had to be changed to
keep a Repo rather than a tarball URL.

Refactoring: add listInstalledPackages, and use that everywhere instead of getInstalledPackages with lots of arguments that all come from ConfigFlags.

But it does not work yet as it need to be updated to take configurations into
account. At the moment it thinks no packages have any dependencies since
it does not finalise the configuration for the package.