We aren't enumerating arguments but values for an argument, so the name
should reflect that.
This will be important as part of #1807 when we have more specific
attribute names.
The error was when doing `#[clap(arg_enum, default_value_t = ...)]`.
Good example of why we should minimize `use`, at least in tests
(besides reducing merge conflicts, code churn, etc).
While I'm unsure how much type specialization we should do, we
intentionally have the `arg_enum` attribute for doing special behavior
based on it, so let's take advantage of it.
Fixes#3185
