UHC's version of registerPackage is the only one that makes use of the
PackageDescription and inplace :: Bool args, and it's quite wrong for
doing so. Registering a package should depend on the content of the
InstalledPackageInfo and the PackageDBStack to register into and the
Compiler to register with. It should not depend on the original source
PackageDescription, and should not need a separate inplace arg. The
location is determined by the PackageDBStack. UHC was not following
this pattern and thereby forcing the general compiler independent
registerPackage to take annoying and unnecessary arguments.

With this patch, the register location is determined by the
PackageDBStack. The source package id also comes from the
InstalledPackageInfo rather than the source PackageDescription.

This patch does not yet change the registerPackage type.