Revert "Fix three bugs with fake-map implementation for PackageIndex."

This reverts commit f59bab102de3663c957b735a8d963d19f0fbde2e.

Revert "Switch InstallPlan over to using IPID-indexed PackageIndex."

This reverts commit 6465d174cbec5cef676b57a409d8f8f310a2b04e.