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

[epage]

What does this PR try to resolve?

When you have

[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

How to test and review this PR?

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r? @ehuss

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[ehuss]

Thanks!

[epage]

failures:

---- cache_lock::mutate_then_download stdout ----

thread 'cache_lock::mutate_then_download' panicked at C:\a\cargo\cargo\crates\cargo-test-support\src\lib.rs:1618:13:
test did not finish within 10 attempts (1.0028045s total)

---- cache_lock::mutate_then_shared stdout ----

thread 'cache_lock::mutate_then_shared' panicked at C:\a\cargo\cargo\crates\cargo-test-support\src\lib.rs:1618:13:
test did not finish within 10 attempts (1.0022253s total)


failures:
    cache_lock::mutate_then_download
    cache_lock::mutate_then_shared

Second time I've seen this recently