Use fallback instead of pkg-config for gazebo discovery

[simonschmeisser]

Currently gazebo_ros depends on gazebo_dev which depends on libgazebo11-dev in order to use the pkg-config file included there to locate the setup.sh (see #323 )

libgazebo11-dev however also depends on libogre-1.9-dev which conflicts with libogre-1.12-dev which rviz would like to use for focal (see ros/rosdistro#24448 for discussion)

This PR adds a fallback to /usr/share/gazebo as this is the location used in the packages and therefore allows to drop the runtime dependency on gazebo_dev

[chapulina]

libgazebo11-dev however also depends on libogre-1.9-dev which conflicts with libogre-1.12-dev which rviz would like to use for focal

That sounds like a no-go to me. We want users to be able to develop Gazebo and RViz plugins side-by-side on Noetic, even if each individual plugin is not compiled against both.

[simonschmeisser]

Nobody stops you from doing that, it just requires compiling either gazebo or rviz from source with either libogre-1.12-dev or libogre-1.9-dev installed. rviz will stay compatible with ogre-1.9 as only that is included in Debian Buster.

[chapulina]

it just requires compiling either gazebo or rviz from source

That's already possible if someone really wants to use RViz with 1.12, right? I believe we're talking about the official versions that will be released into rosdistro. And I believe they shouldn't conflict so users have a better out-of-box experience.