Previously, we just added a -inplace suffix, but this will cause
problems when developing multiple packages inplace, and then
installing them.

Also, there was a round of refactoring: registerPackage now takes the
InstalledPackageId as an argument, and generateRegistrationInfo is
exposed for constructing it.  This means that callers of
registerPackage get to munge the InstalledPackageInfo before it is
registered.