fix(add): Report a missing source error for workspace dependencies

When you have
```toml
[workspace.dependencies]
apply.workspace = true
```
You have a source-less workspace dependency with an unused `workspace`
field.

In #13775, we removed support for source-less dependencies but missed
this case.
This mirrors the error message from `toml/mod.rs` that #13775 added.

Other options I considered:
- Making the presence of `workspace` an error in `cargo-util-schemas`
- Validating `workspace.dependencies` in `toml/mod.rs`, even when its not inherited

In both cases, they are behavior changes that and I wanted to keep the
focus of this change small rather than get tied up in what behavior
changes are safe or should be done with an Edition.

The second option could still lead to this panic if we move some of that
error reporting into later stages of Cargo.

Fixes #1398

Author: epage

src/cargo/ops/cargo_add/mod.rs

@@ -516,7 +516,11 @@ fn resolve_dependency(
             let query = dep.query(gctx)?;
             match query {
                 MaybeWorkspace::Workspace(_) => {
-                    unreachable!("This should have been caught when parsing a workspace root")
+                    anyhow::bail!(
+                        "dependency ({}) specified without \
+                        providing a local path, Git repository, or version",
+                        dependency.toml_key()
+                    );
                 }
                 MaybeWorkspace::Other(query) => query,
             }