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.
