per-variant timestamps

Timestamps are currently associated with packages, not variants. This needs to be changed, otherwise we cannot facilitate adding of variants to a package after it's been installed - something we're going to need to do.

No files, tests, or entry points are named. Start by locating where Rez stores package-level timestamps and how variants are added after installation; completion means timestamps are associated with individual variants and a new variant can be added to an installed package.

Written by the indexing model from the issue text.