Error out in case of adding members of Cargo virtual workspace individually in dist-workspace.toml

[alilleybrinker]

See: https://github.com/mitre/hipcheck/actions/runs/11751317704/job/32741130807

In this GitHub Actions error, you can see that each crate in the Cargo virtual workspace is duplicated 14 times. This is because the workspace.members field of the dist-workspace.toml file added each dist-able member of the Cargo virtual workspace individually. When this is processed to identify all distable packages, each call to crate::rust::get_workspace actually identifies all packages in the overall Cargo virtual workspace, and this is run 14 times (one for each entry in workspace.members, resulting in the duplication.

I think the right course of action would be to identify when packages being specified as Dist workspace members are in a virtual Cargo workspace, and producing an error saying to instead specify the Cargo virtual workspace root itself.

[duckinator]

Thank you for digging into this! I agree, telling people to use the virtual workspace itself makes sense.