This patch just adds the state without yet using it. That'll follow in
subsequent patches.

So why add an Installed state? Didn't we just remove the Installed,
Processing and Failed states? Those states were used when we followed
the approach of updating the InstallPlan as a build progressed (whereas
we now do traversals without altering the InstallPlan).

The idea of adding an Installed state now is that we can more usefully
represent the state of the plan when we "improve" the plan with packages
from the store or when we update the plan having checked if inplace
packages are up to date. Currently in these two places we replace
Configured source packages with PreExisting packages. There's a couple
problems with this. Firstly the PreExisting state only contains an
InstalledPackageInfo which means we loose information compared to all
the detail in the Configured source package. This is relevant for things
like plan.json output or other features that want to know the status of
a project. Secondly we have to fake things for executables since they
are not properly represented by InstalledPackageInfo.