Adjust packaging of catalog games into Python build.

[tturocy]

This makes a few adjustments to how we include the catalog games in the Python build.

Support for 'package data' in Python isn't great currently, and further we have a desire to keep the catalog
games elsewhere
in our tree and not embedded inside the
Python source.

Our solution is this:

• We have a custom build class, which is responsible for copying the contents of the catalog directory from where it lives into the build directory.
• We put this in "catalog_data" to avoid clashes with e.g. just having a "catalog" source file or module.

The extra build step is moderately annoying but
straightforward.
However, it avoids e.g. pip thinking that the
catalog data is Python code and complaining
about the package not being listed.

Closes #823.

[tturocy]

@edwardchalstrey1 Here's a candidate solution to the issue. I think it's coherent and it's resolved the warning for me. It is slightly flddly but that's the cost of us wanting to be able to pull the catalog data from elsewhere (which we will definitely want to do, as someday it will possibly be its own repo which we e.g. refer to by a submodule!)

Question: Do we need to memorialise this anywhere in the developer documentation?

[edwardchalstrey1]

Great, this solution works for me!

Regarding docs, the process for updating the catalog is unchanged, however I did add the header "The catalog module" before our discussion, which is now out of date. This could either be updated or simply removed, depending how useful you think it is for devs on the project to read vs just inspecting the codebase.