how does roslaunch $(find ...) actually work?

edit

Short version of the question: When using installspace, where does $(find pkg) look for files? rospack find pkg returns the path to the share folder, but then I realized that we are able to do for instance $(find pkg)/scripts/test.py although test.py is installed in the lib folder. How can this work?

Long version of the question: I have the following case (very common in the ROS community):

Package A:

A
| - CMakeLists.txt
| - package.xml
| - launch
       | - test.launch
| - scripts
       | - test.py

CMakeLists.txt content:

# Regular Catkin CMakeList.txt
install(PROGRAMS
  scripts/test.py
  DESTINATION ${CATKIN_PACKAGE_BIN_DESTINATION}
)

install(DIRECTORY launch
  DESTINATION ${CATKIN_PACKAGE_SHARE_DESTINATION}
)

test.launch content:

<launch>
    <param name="my_param" command="$(find A)/scripts/test.py"/>
</launch>

test.py content is not meaningful.

After using installspace (or cross-compiling with bitbake using meta-ros repository) the file structure will look like:

$(ROS_DIR)
| - share
        | - A/launch/test.launch
| - lib
        | - A/test.py

I always thought that $(find pkg) worked the same as rospack find pkg but apparently not, as the second returns $(ROS_DIR)/share but the above launchfile is able to run the script in the lib folder. Therefore, how does $(find pkg) work, how does it resolve the corresponding path to use?