Both `name` and `alt_registry_key` are mainly used for displaying.
We can safely merge them into one enum.
However, `alt_registry_key` is also used for telling if a SourceId is
from `[registries]` so we need to provide functions to distinguish that.

`registry_key` should always be `crates-io` when `is_crates_io()` is true

This also removes the test verifying `Debug` output,
whose format doesn't need to be guaranteed.