Move sandbox types under a dedicated package

[sameo]

Description of problem

In order to fix/implement #1059, we need to start putting all internally shared structures under an internal types package. Without doing so, breaking the code into separate packages will create circular dependencies (e.g. sandbox needs the internal store package, which needs some of the sandbox specific structures).

There is no need for moving all internal types in one go. This can be done iteratively as we move pieces of the current code into their own packages. The first big piece of this work is to move the sandbox shared structures under an internal types package.