uninstall -all shouldn't attempt to uninstall IPM itself.

[isc-shuliu]

Currently, running uninstall -all will attempt to uninstall all packages, including IPM itself.

zpm:USER>uninstall -a

Uninstalling zpm
[USER|ZPM]	Clean START
[USER|ZPM]	Unconfigure START
[USER|ZPM]	Unconfigure SUCCESS
[ZPM]	Clean FAILURE
Error uninstalling zpm: ERROR #5002: ObjectScript error: <CLASS DOES NOT EXIST>%Clean+141^%IPM.Lifecycle.Base.1 *%IPM.StudioDocument.Module

<CLASS DOES NOT EXIST>DisplayError+2^%IPM.Main.1 *%IPM.Repo.UniversalSettings
USER 6d1>

We should explicitly exclude IPM from the uninstall list and there should be a separate implementation for uninstalling IPM itself.

[isc-shuliu]

Also, unless this is already implemented, we need to figure out the uninstallation order based on the dependency graph (we might need to do a topological sort first)