Revise self-hosted command line interface

Commands are now separated more precisely from one another. Arguments
are parsed mostly using a custom argument parser instead of manually.
This should be on parity feature-wise with the previous main.zig but
adds a few extra code-paths as well that were not yet implemented.

Subcommands are much more prominent and consistent. The first argument
is always a sub-command and then all following arguments refer to that
command. Different commands display there own usage messages and options
based on what they can do instead of a one-for-all usage message that
was only applicable for the build commands previously.

The `cc` command is added and is intended for driving a c compiler. See #490.
This is currently a wrapper over the system cc and assumes that it
exists, but it should suffice as a starting point.

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