Warn about missing features in workspace crates.

Repro

https://github.com/LouisGariepy/workspace_crate_missing_features.

Story

User creates a project with crate A the requires dependency D with feature foo.

Later, the user creates crate B, which also needs D, but they forget to add feature foo. Due to the way features are resolved in workspaces, they can still use items gated behind foo (because of crate A's enabling foo).

If the user tries to build crate B standalone (cargo build -p B), then Cargo will report missing items. Same if they decide to move crate B out of the workspace.

Resolution

R-A could give a warning about missing features in workspace crate where a feature is enabled only by other crates. This avoid pitfalls over rebuilding a single crate in a workspace, or moving a crate out of a workspace.

I don't know of feasible that is though, knowing which features are provided by other workspace crates but not declared locally seems difficult.

Start with the reproduction project at https://github.com/LouisGariepy/workspace_crate_missing_features and compare the workspace build with cargo build -p B standalone. Investigate how workspace feature resolution distinguishes features enabled by other crates from features declared locally. Done means a warning reliably identifies missing local features without producing false positives.