Fix generation of Cargo completions for ZSH (issue #1821)

[burjui]

When sourceing the original completion file, the #compdef cargo line at the start is ignored, and ZSH does not autoload the completions, so rustup must put it the generated _cargo as well.
_{I guess rustup does not create a symlink instead, because that would be less portable.}

[kinnison]

I do not use zsh completions with cargo; but I can appreciate others might -- is there anyone who can confirm if this is sufficient to make things work?

[ehuss]

I don't normally use zsh, but I tried this out and it seems to work (zsh 5.7.1). Looks like it needs a rustfmt though.

[burjui]

@ehuss Done.