lib: port CLI handling to clap v3

[cgwalters]

This reworks CLI handling logic, porting to latest clap v3.

Closes: #373

[lucab]

I'm doing some other clap-v3 porting, so I can take this over too.

[lucab]

For a start, I rebased the branch and brought it to green CI state.
There are tons of (deprecation) warnings right now though, I'd like to sort them out here.

[lucab]

Ok, porting here is completed and this is ready for review.
Please note that this is an API breaking change.
Also, clap v4 just got released and it is going to be another API break. We may want to consider switching to that before tagging a release here.

[cgwalters]

Yeah in some conversation over at https://www.reddit.com/r/rust/comments/xqiqfy/clap_400_a_rust_argument_parser_is_released/ it seems like some people were aware a v4 was coming and held off porting from v2. But, I guess since we're doing this we might as well do v4?

Potentially related to this, we could split off the CLI code into a separate crate with its own semver churn. Definitely some cost/benefit to that.

Anyways...yeah I think I'd say we should go v4, but I haven't tried the port either myself. I can do it if you like.

[lucab]

@cgwalters when we land this, the clap-v4 update should be just a Cargo manifest bump (as this PR already gets rid of all the planned deprecations).

[cgwalters]

LGTM as is but I can't approve this as it's my own PR so...feel free to approve 😄

[lucab]

LGTM (feels like cheating) 😄

[cgwalters]

Please note that this is an API breaking change.

Oh, right. Well, we forgot to bump semver for this. Urgh. Well...ok. In practice I can't imagine anything is using this API except rpm-ostree.

If something is, well, I can't imagine we're causing them that much pain.