Rollup merge of #107470 - kadiwa4:bootstrap_cleanup, r=albertlarsan68

Small bootstrap improvements

- i/o-less check for `xz` availability
- cache result of NixOS detection
- load correct `bootstrap` module even when a package of that name is installed
- no `-W semicolon_in_expressions_from_macros` – it is warn-by-default
- one type per variable (making dynamic typing less confusing)
- integrate python-side `--help` flag into the argument parser (makes `-hv` work as a short form of `--help --verbose`)

I even checked that it works with Python 2.