Allow Arup internal packages to depend on other Arup internal packages

[brynpickering]

With an Arup internal conda channel at https://packages.arup.com/conda we could feasibly have conda requirements files include those packages. This is fine when working locally (just include -c https://packages.arup.com/conda when creating the environment / installing the dependency). It doesn't work in CI/CD as the github runners don't have access to https://packages.arup.com/conda. The Arup packages runner does, but there's a few showstoppers here:

1. We can only run linux tests as the packages runner is a linux machine
2. We might overload the resources assigned to the runner if we the tests are too meaty

I'm mostly opening this issue to clarify to others why we can't (currently) have internal packages dependent on other internal packages if we want to leverage any CI/CD on GitHub.