Feature: SiLA code_generator add convienient cli argument "--features-directory", similar to the --package-directory of the add-features command

Currently adding a whole directory of features to be used by the code_generator requires to use the my_feature_dir/*.sila.xml argument. It would be a little bit more convenient (and a kind of symmetric to the add-features command), if we could add a --package-directory argument, like:

sila2-codegen -n my_sila_package --features-directory my_sila_features