Add consider_namespace_packages=False (backport #766)

[mergify[bot]]

Fixes https://github.com/ros2/launch/issues/765. That's a quick patch to avoid an error but I don't know if it's the "cleanest" solution, please advise.

Needs backporting humble and iron

---

This is an automatic backport of pull request #766 done by Mergify.

[clalancette]

CI:

• Linux
• Linux-aarch64
• Windows