ci: Pin extra test dependencies for generated test suites (#6696)

Pin additional test environment dependencies. These additional dependencies are provided in the `deps` entry for a package in `populate_tox/config.py`.

The `fetch_package_dependencies()` function accepts the new `install_additional_targets` boolean parameter. If provided, the packages in the `deps` entry are added to the targets in the `pip` dry run that determines dependency pins. `DEPENDENCIES_CACHE` is now keyed by Python version and a hash of the installation targets.

Author: alexander-alderman-webb