_cargo: Add cargo package completion

[tesuji]

Continuation of #7060
r? @ehuss

[tesuji]

Ran on cargo repository:

[tesuji]

Ping @ehuss

[ehuss]

This seems kinda buggy. For example, the completion includes backslashes and whitespace sometimes.

Also, reading the manifest is a poor way to find the valid values for -p.

I personally would prefer to avoid spending any time enhancing these completion files (anything beyond trivial changes) and wait for #6645. If you want to help with #6645, that would be appreciated! The approach for that will be substantially different, using rust code to drive the completions which should be much easier to implement, test, etc.

[tesuji]

Could you give an example how is this bugged?

[ehuss]

Doing completion in cargo's own repo: > cargo build -p=clap\ .
This is on macOS: zsh 5.7.1 (x86_64-apple-darwin18.2.0)

But as I said, I'd rather not spend time reviewing or maintaining this code, since we intend to remove it.

[alexcrichton]

I'm going to close this since it's been quiet for a bit and due to @ehuss's previous comments.