Scheme code coverage can target external and SPM projects

Project schemes can now include code coverage targets from any project in the workspace, including packages produced by SPM projects and external targets, by pointing at them with an explicit TargetReference(projectPath:target:). Tuist was previously rejecting this valid configuration as “isn’t defined in the project”. Local code coverage references keep their existing validation, and unresolved references are still caught by the workspace-level graph linter.