* Flesh out the formerly incomplete ItemSlug class
* Convert all uses of text item slugs to use ItemSlug
* Move the ItemSlug class under models where it properly belongs
* Avoid the use of `slug` as a local variable or item attribute
wherever possible to avoid confusion between various kinds of slugs
* Merge the InventoryParser class into the Inventory class as it is
unlikely to need to support multiple versions
* Simplify the ModPack/Mod/ModVersion interfaces by replacing the use
of various "helper" methods with equivalent use of more generic
methods
* Update the various methods of ModPack/Mod/ModVersion to be able to
work with qualified or unqualified ItemSlugs
* Remove the unused BaseCollection class
* Add tests for ItemSlug and update all other tests to reflect
all the other changes
