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).