Align package checkout with colcon workspace convention (src folder)

ROS packages are usually checked into a repository either at the root level (for a single package) or in directories that directly contain the packages (for multiple packages). A colcon workspace, however, is conventionally structured with a src folder that contains all package directories.

Currently, builds are executed directly from the repository root. To better follow the colcon workspace convention, packages should be checked out into a src path, with the workspace built from there.