Global constants that get modified during precompilation don't work

The `@theory` macro includes a line to register each theory module, indexed by the data of the theory.

```
const GAT_MODULE_LOOKUP = Dict{ScopeTag, Module}()
```

This is convenient when one is working with GATs rather than the associated theory modules but then need to do something with the theory module, e.g. using a theory morphism to do a model migration.

However, when testing this with Catlab, this no longer seems to work. Catlab adds a new theory, `T`, and during precompilation one can check that `T` is in `GAT_MODULE_LOOKUP`. But if one loads up GATlab and Catlab in the REPL, `T` is no longer in the cache (only the theories added in GATlab are there).

This might be relevant https://discourse.julialang.org/t/mutating-global-variable-during-precompilation/51478/3 \

In the immediate future, one workaround could be to only use `@theorymap` modules (which have the dom and codom modules) for migration.