The enviroment stored in the JLSO means we always have access to the right packages.
Bu they need to be usinged (or at least Base.requireed) to have the JLSO correctly deserialize any types from them.
Else you get a UInt8 array.

Ideally we would workout which are actually used in the JLSO for a particular object and be able to using them.

A work around is to load everything.
It is a bit gross but the following works (I think it needs julia 1.4)

using Pkg: Pkg
eval(Expr(:using, (Expr(:., n) for n in Symbol.(keys(Pkg.Types.Context().env.project.deps)))...))