Only include targets of packages that are workspace members

Only include targets of packages that are workspace members

CargoWorkspace's package list includes packages that are path
dependencies, even if those packages aren't actually members of the
cargo workspace. As a result, rust-analyzer's runnable finder, which
returns the target from the first workspace that has a matching package,
may select the wrong working directory, causing runnables to fail, e.g.,
```
error: package `root` cannot be tested because it requires dev-dependencies and is not a member of the workspace
```

To fix this, we filter out packages that aren't members of the workspace
when searching for targets.

Fixes #7764