This is a step towards #3309. We want to make longs and long aliases
more consistent in how they handle leading dashes. There is more
flexibility offered in not stripping and it matches the v3 short
behavior of only taking the non-dash form. This starts the process by
disallowing it completely so people will catch problems with it and
remove their existing leading dashes. In a subsequent breaking release
we can remove the debug assert and allow triple-leading dashes.
This is inspired by cargo which allows you to run `cargo test --test`
and it will list the possible tests (obviously we can't support that atm
because that requires a lot of runtime processing). When we do have a
static list of possible values, we can at least show those.
Fixes#3320
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.
