Allow scheme code coverage targets from external projects

Project schemes can now include code coverage targets from any project in the workspace, including Swift Package Manager packages, by referencing them with an explicit TargetReference(projectPath:target:). Previously, Tuist rejected these configurations during project linting and tuist generate failed, even though Xcode supported them natively. Local target validation is preserved (typo detection still works), and invalid cross-project references are still caught by workspace-level graph validation.