catkin part error when built after python part

[Ruddick Lawrence]

When snapping the attached example with the snapcraft master branch (currently https://github.com/snapcore/snapcraft/tree/52e03e5e46d88809f706fb4f45e6556710d576ba), it fails with the error:

Unable to find user site packages: /home/ruddick/Downloads/snapcraft_ros_example/parts/ros-project/install/lib/python2*/site-packages

The cause is the following new code since 2.34: https://github.com/snapcore/snapcraft/blob/52e03e5e46d88809f706fb4f45e6556710d576ba/snapcraft/plugins/catkin.py#L652-L657

It fails because self._pip.is_setup() uses stage to see if pip is installed, which it was in the pip-install part, but _python.generate_sitecustomize() operates on parts/ros-project/install, which does not have any python code or site-packages directory.

[Kyle Fazzari (kyrofa)]

Good catch Ruddick, thank you. Indeed, there's no reason to generate a sitecustomize unless pip dependencies were pulled in the Catkin part itself, which it's not. Fixing now.

[Kyle Fazzari (kyrofa)]

Proposed fix is here: https://github.com/snapcore/snapcraft/pull/1717