`Arg::exclusive` is just another way of defining conflicts, so a
present-exclusive arg should override required like other conflicts.
Instead of going through the message of enumerating all other arguments
as exclusive, I shortcutted it and special case exclusive in the
required check like we do with conflicts. The big downside is the
implicit coupling between the code paths rather than having a consistent
abstraction for covering conflicts.
This isn't a breaking change because if someone defined an exclusive arg
as a sibling to a required arg, the exclusive arg could never be used,
it always errored, and so no valid application can be written with it.
Fixes#3595
We were only tracking the last value source (default, env, cli, etc).
This works for args because they only come from one source. Groups
however can come from multiple sources and this was making us treat a
group with a default value as being completely from defaults despite
some values maybe being from the commandline.
We now track the highest precedence value for a group.
Fixes#3330
This is prep for moving the derive tests. Besides organizing the test
folder for each API, this should reduce link time at the cost of
re-compiling more when a test changes.
