When registering, choose the InstalledPackageId.

 - When registering inplace, use "foo-1.0-inplace"

 - If this isn't GHC, just use "foo-1.0-installed"

 - When installing a package with GHC, call
   Distribution.Simple.GHC.libAbiHash to get the hash, and
   use "foo-1.0-<hash>".