Help message for x.py

[fpoli]

The x.py script interprets its subcommands and arguments using some custom parsing rules, but discovering them is difficult because the script does not print any help message.

• x.py and x.py --help should print a proper help message, listing the possible subcommands and a 1-line description for each of them. This can be done by replacing the parser with one defined using the argparse library.
• The arguments after a subcommand should be passed as they are to the corresponding command. For example, x.py build --help should execute cargo build --help instead of printing a help message from Python.
• x.py should not require to install new Python dependencies (so, no click for example).