Galactic: 'No rule to make target' for custom messages

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This is for Galactic / Ubuntu 20.04.

I'm new to ROS2 and am getting a consistent 'No rule to make target' error when trying to build packages that produce interfaces via the rosidl_generate_interfaces command. Here is an example for the turtlesim package.

--- stderr: turtlesim                               
make[1]: *** No rule to make target 'turtlesim__py/CMakeFiles/turtlesim__py.dir/all', needed by 'CMakeFiles/turtlesim__python.dir/all'.  Stop.
make[1]: *** Waiting for unfinished jobs....
make: *** [Makefile:146: all] Error 2
---
Failed   <<< turtlesim [5.25s, exited with code 2]

I've dug into the relevant build files to investigate. In turtlesim/CMakeFiles//Makefile I see line 429 where the error is thrown:

CMakeFiles/turtlesim__python.dir/all: turtlesim__py/CMakeFiles/turtlesim__py.dir/all

The error is that the dependency turtlesim__py/CMakeFiles/turtlesim__py.dir/all does not exist. However, I see farther down on line 656 that this target is created:

/home/blake/ros2/build/turtlesim/turtlesim__py/CMakeFiles/turtlesim__py.dir/all

The difference being that the target uses an absolute path when declared, but a path relative to the makefile when called as a dependency. The relative path is not recognized as equivalent to the absolute path.

Can anyone explain the cause of this issue? I expect it must be some kind of environment configuration problem, but my env variables all appear correct per the ROS2 tutorials.