Dropping these will help simplify a lot, including removing of
occurrences.

These come at the cost of the derive not yet supporting types that impl
`From`.

This shouldn't be needed anymore now that this is effectively the new
behavior for the non-deprecated actions.

This was briefly talked about in
clap-rs#2627 but I wasn't familiar
enough with the implementation to know how safe it is.  Now, maintainrs
and users can be more confident because they are explicitly opting into
it.

See also clap-rs#3795

This mostly exist for
- Knowing of the value came from the command-line but we now have
  `ArgMatches::source`
- Counting the number of flags but we now have `ArgAction::Count`

This is needed for deprecate `multiple_occurrences`

For programs opting into the clap v4 behavior (with `action` or
`value_parser` attributes), we'll no longer generate a
`multiple_occurrences(true)` call in preparation for deprecating
`multiple_occurrences`.  See clap-rs#3772.

Fixes clap-rs#3772