Building from source, rqt fails

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Hi,

I am attempting to install ROS 2 Galactic from source on Ubuntu 20.04 x86_64 according to the instructions here: https://docs.ros.org/en/galactic/Inst...

I have reached this step:

colcon build --symlink-install

The build fails with the following output:

--- stderr: rqt                                                                                                                                             
error: Multiple top-level packages discovered in a flat-layout: ['share', 'resource'].

To avoid accidental inclusion of unwanted files or directories,
setuptools will not proceed with this build.

If you are trying to create a single distribution with multiple packages
on purpose, you should not rely on automatic discovery.
Instead, consider the following options:

1. set up custom discovery (`find` directive with `include` or `exclude`)
2. use a `src-layout`
3. explicitly set `py_modules` or `packages` with a list of names

To find more information, look for "package discovery" on setuptools docs.
---
Failed   <<< rqt [13.0s, exited with code 1]

Why is this happening? What can be done to fix it? Thanks.