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.
Since usage parser and yaml are on the way to being deprecated (#8, #9),
doing a rename also seems excessive, so rolling it back.
Past relevant PRs:
- clap-rs/clap#1157
- clap-rs/clap#1257