Install plan improvement is the process of replacing configured source
packages with installed instances from the store. Originally we did this
by reading the ghc-pkg db to get the InstalledPackageInfo for all the
packages in the store. We had to do that because when we replaced
configured source packages by installed instances we used the
PreExisting constructor which requires an InstalledPackageInfo, which we
get from the installed package db. But now that we no longer use
PreExisting for packages from the store we also no longer need the
InstalledPackageInfo. All we need is a set of UnitIds. Also, once
support for depending on executables was added then we needed a way to
do plan improvement for executable packages/components. We did this by
the simple approach of grabbing the dir listing for the store and using
that as a set of UnitIds.

So this patch extends the approach we use for executables and uses it
for all packages. This means we no longer load the package db for the
store. Note that we still have to create the package db in the store.

This also relates to the locking protocol in the store. The goal for the
store is to be able to access and update it concurrently. The locking
protocol will include checking membership by checking if the directory
entry for the package is present. So this patch gets us to the right
point for the reading side, leaving the writing side to do.