brain_pytest and brain_gi should not import their underlying module (gi and pytest)

This breaks the static analysis contract since we're now importing third party libraries to build AST trees from them. While this makes sense for plugins, we shouldn't do this for core astroid, as it means we're now breaking the contract by importing live modules. Note that we currently do that with `raw_building`, but we only do that by default for builtin modules, while for third party modules we rely on `--extension-pkg-whitelist` option.