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A ROS package is defined as a directory with a package.xml file. Similar to how a CMake package (CMakeLists.txt) and Python package (setup.py) file is defined. Everything recursively under that directory is part of that package. As a consequence packages can't be nested.

Either you place multiple packages beside each other into sibling directories (see e.g. the ament_lint repository which contains numerous packages in sibling directories on the top level) or you keep adding content (like additional Python modules) to an existing package (see e.g. the launch package which contains many Python module below the launch subdirectory).