add configure-time flag to skip parent_prefix_path

[dirk-thomas]

Related to ros2/ros2#929.

The diff is best looked at while ignoring indentation changes: https://github.com/ament/ament_package/pull/115/files?w=1

No CI since this isn't covered by it anyway but needs to be manually verified.

[dirk-thomas]

Using this modified template the setup.py file for e.g. Foxy can be generated with this logic:

import ament_package.templates
p = ament_package.templates.get_prefix_level_template_path('setup.sh.in')
print(ament_package.templates.configure_file(p, {'CMAKE_INSTALL_PREFIX': '/opt/ros/foxy', 'SKIP_PARENT_PREFIX_PATH': 'SKIP_PARENT_PREFIX_PATH'}), end='')

[nuclearsandwich]

Passes a visual inspection. Getting set up to test these changes in a workspace.

[nuclearsandwich]

I built modified ament_package and ros_workspace debs, using the former when building the latter, and then pushed that modified ros_workspace deb to the container I did my tests in with this result:

root@ros2-focal:~# time bash -c '. /opt/ros/foxy/local_setup.bash'

real    0m0.157s
user    0m0.124s
sys     0m0.038s
root@ros2-focal:~# time bash -c '. /opt/ros/foxy/setup.bash'

real    0m0.158s
user    0m0.138s
sys     0m0.025s

[dirk-thomas]

Fast-forwarded the squashed commit from master to foxy.