fix: move command_line to separate crate

[RobertMueller2]

Closes: #303

Currently, by including command_line.rs in build.rs, the former gets snatched into crate "build_script_build".

This has a number of funny and unintuitive side effects like dependencies not working for the file but the build dependencies.

Here we move command_line.rs into a separate "cli" crate. Technically, this might not be much of an improvement as we still have to add dependencies to the new crate, but it makes the behaviour more obvious.

[RobertMueller2]

I'm kinda thinking the clap documentation should point this out, it does suggest include!. (Which caused the same result in my test)

Or maybe it's a shortcoming of the rust compiler. My expectation would be that a source file could be part of both, of course in that case its dependencies would have to be in both dependencies AND build-dependencies. But perhaps my understanding of this isn't good enough at this point ;)