An example of how to organize monorepository that provides SPM modules for the outside world. See https://github.com/kukushechkin/sampleProject for how to use those dependencies.
Unfortunetely, SPM explicitly forbids mixing of versioned and unversioned dependencies, that's why components cannot be Swift Packages inside another big Swift Package. See .revisionDependencyContainsLocalPackage in https://github.com/apple/swift-package-manager for more details.
Conclusion? No way. While root Package.swift is ok for rebuilding modules, it will not be able to handle deendencies.
What will work — make root Package.swift with path modules dependencies, clone it and use as path dependency in your project.