Warn about security advisories for cratres being added with cargo add

[epage]

Problem

A user can add a crate with a security advisory and not know it unless they know of the third-party cargo audit, install it, and run it.

Proposed Solution

Integrate cargo audit checks into cargo add when adding a new registry dependency

Notes

Inspired by conversation on zulip about checking it in cargo

It looks like we

• Fetch a repository
  • rustsec::repository::git::Repository::default_path
  • rustsec::repository::git::DEFAULT_URL
• Load the database from repo
• Run a query on the database
• Report it to the user (example from cargo-audit)

We might be blocked on rustsec/rustsec#490

[epage]

See also killercup/cargo-edit#512