Make .cargo/credentials a subset of .cargo/config

Make .cargo/credentials a subset of .cargo/config

Previously, .cargo/credentials looked like

```toml
token = "..."

[my-registry]
token = "..."
```

And was simply merged into the `registry` block of .cargo/config. This
meant that custom registry tokens were under
`registry.my-registry.token` rather than `registries.my-registry.token`
which is where the index was located, and that you couldn't have a
custom registry named `token` or it'd conflict with the token for the
default registry.

This commit changes things such that .cargo/credentials has the same
layout as .cargo/config, but only contains token values. For backwards
compatibility, we move `token` to `registry.token` when parsing.