remove getattr from SitePackages (#6867)

`__getattr__` can be cute and all, but it's unhelpful for typechecking
(and human code readers): suddenly any attribute on an object might be
valid, and might return any type at all.

mypy and the unit tests agree that no use is made of the `__getattr__`
on `SitePackages` anyway, so removing it is simply making it clear that
there is no magic.