package_release: refactoring

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.