girepository: Also store GType cache misses
There are notably 3 classes of GTypes where a girepository lookup might fail:
- GTypes from private interfaces in public objects (eg. MetaCullable in mutter)
- GTypes for private base objects with public interfaces (eg. GLocalFile in GLib)
- GTypes registered from the language, and presumably not coming from the GIR
It is moot to look for those over and over again, and a full lookup can be taxing if looking up for a method/property on objects with those characteristics.
It seems we can cache the misses too, so next lookups are just as quick as an introspected GType.