Naga is now a workspace with `naga` and `cli` as its two members. The default
package for cargo commands is `cli`, so that `cargo run` will just run the CLI.

However, this has a few unexpected consequences:

- Now `cargo test` will just try to run `cli`'s tests, of which there are none.
  Adding `"."` to the `default-members` list in the workspace's `Cargo.toml`
  seems to fix this, without breaking `cargo run`.

- Even with `"."` added to `default-members`, `cargo test` will build `naga` by
  default with the features requested for it in `cli/Cargo.toml`: all the front
  and back ends, but no `serialize` or `deserialize`. This means that our CI job
  meant to verify no-feature builds isn't doing that job any more. We need to
  pass `--package naga` to `cargo test` to make it test naga directly.