Select specific workspace package in `--git` overrides

[eopb]

Problem

When creating a git override with cargo-overrides --git <uri> flag, a URL is provided for the git repository. If that git repository happens to contain a workspace, cargo-override will use the default package in that workspace.

For example, since sqlx defines a default package at its root, it'll be that crate that gets patched when cargo override --git https://github.com/launchbadge/sqlx is used. It is not currently possible to patch an of the other sqlx crates with --git, such as sqlx-core.

Crates like tokio and tracing, don't contain any default package in their workspace root, so aren't possible to patch at all. cargo override --git currently always fails for those.

Suggested solution

First we need to ensure that git patches of crates, without a default package, are possible with Cargo. I have not tested this so am not 100% certain it works.

Once that is done, we can add a -p <pkgid> flag that can be used with --git to specify the package we wish to use in the patch.

[eopb]

It could also make sense to generalise this change to work with --path also, since the path could also contain multiple packages in a workspace.

This is also a nice opportunity to have improved error/help messages, where we print out package IDs of the potential packages that can be passed into -p <pkgid>

[epage]

My recommendation would be to act like cargo.

--path maps to a PathSource which expects a package at that location.

--git maps to a GitSource which wraps a RecursivePathSource. I would patch everything found within the GitSource that is present in the Resolve, warning when a referenced package name is ambiguous.