use argparse & small refactoring

[Artturin]

no functionality changed

[Artturin]

2>/dev/null nix run ".#default" -- --help
usage: hydra-check [-h] [--url] [--json] [--short] [--arch ARCH] [--channel CHANNEL] PACKAGES

positional arguments:
  PACKAGES

options:
  -h, --help         show this help message and exit
  --url              only print the hydra build url, then exit
  --json             output json
  --short            write only the latest build even if last build failed
  --arch ARCH        system architecture to check
  --channel CHANNEL  Channel to check packages for

Other channels can be:
    unstable  - alias for nixos/trunk-combined (Default)
    master    - alias for nixpkgs/trunk
    staging   - alias for nixos/staging
    19.03     - alias for nixos/release-19.03
    19.09     - alias for nixos/release-19.09
    20.03     - alias for nixos/release-20.03
    nixpkgs/nixpkgs-20.03-darwin    - verbatim jobset name

Jobset names can be constructed with the project name (e.g. `nixos/` or `nixpkgs/`)
followed by a branch name. The available jobsets can be found at:
* https://hydra.nixos.org/project/nixos
* https://hydra.nixos.org/project/nixpkgs