Fix re-init

[abelmarnk]

Each time the init command is run on an already existing workspace the workspace(explicit as "") is added to the Cargo.toml each time, causing errors, this change makes a re-init to be a no-op.
Note: In the case where new packages were added to the workspace since it was initially initialized, if the new packages reflect in the Cargo.toml then the only thing that happens is what is described above, in the case where it doesn't, the init command fails with a cargo-metadata error, so except for the change described above the init command stays the same.

[pksunkara]

I am willing to accept this change, but can you please add appropriate tests? Thanks.

[abelmarnk]

I've added more tests, i also renamed the tests for clarity since there are more cases to consider.

[pksunkara]

Looks good, just minor issues:

1. Fix formatting using cargo fmt
2. The error of the failing test cannot use snapshot. You can do it similarly to how I have done in a few other places. For ex:

    assert!(err.contains("error: the workspace already contains a package with this name"));

[abelmarnk]

Thanks for the feedback, I've made the changes.

[pksunkara]

Thanks for the high-quality PR. Really appreciate it. 🫰