Improve readability of package_release.py and its submodule
generate_release_metadata.py.
Add describing comments throughout the file.
Remove duplicate code from functions. Simplify them, so that they do
what their names say (and nothing more).
Rename variables and function arguments to make their purpose clearer.
Harmonize variable names with their corresponding command line arguments.
Add pydoc for functions.
Add type declarations to function arguments.
Generate MD5 and metadata files in the source directories so that they
can be copied along with the rest of the files.
Make output-dir an argument that can be passed multiple times instead
of hard-coding two dirs with certain meanings.
Remove LOCALDIR variable from package.env, because it is not used
anywhere (only occurence was job class ".uploadsdkftp" which wasn't
used anywhere either).