Fix rosidl_generator_py assuming incorect library names (#149)

rosidl_generator_py was assuming all interface packages being depended
upon call their interface generation target name "${PROJECT_NAME}", but
that's an incorrect assumption. The target name is a required argument
to rosidl_generate_interfaces. This fixes it by exporting the python
library target using an existing CMake macro for that purpose called
rosidl_export_typesupport_targets() and amen_export_targets()

I works by adding a variable
"${PROJECT_NAME}_TARGETS__rosidl_generator_py" which is set when the
interface package is `find_package()`d. That variable contains the
targets generated by `rosidl_generator_py` so that downstream interface
packages can depend on it.

Signed-off-by: Shane Loretz <sloretz@osrfoundation.org>

rosidl_generator_py/cmake/rosidl_generator_py_generate_interfaces.cmake

@@ -159,7 +159,10 @@ macro(set_lib_properties _build_type)
     RUNTIME_OUTPUT_DIRECTORY${_build_type} ${_output_path})
 endmacro()
 
-set(_target_name_lib "${rosidl_generate_interfaces_TARGET}__python")
+# Export target so downstream interface packages can link to it
+set(rosidl_generator_py_suffix "__rosidl_generator_py")
+
+set(_target_name_lib "${rosidl_generate_interfaces_TARGET}${rosidl_generator_py_suffix}")
 add_library(${_target_name_lib} SHARED ${_generated_c_files})
 target_link_libraries(${_target_name_lib}
   ${rosidl_generate_interfaces_TARGET}__rosidl_generator_c)
@@ -174,7 +177,7 @@ target_link_libraries(
   ${PythonExtra_LIBRARIES}
 )
 target_include_directories(${_target_name_lib}
-  PUBLIC
+  PRIVATE
   ${CMAKE_CURRENT_BINARY_DIR}/rosidl_generator_c
   ${CMAKE_CURRENT_BINARY_DIR}/rosidl_generator_py
   ${PythonExtra_INCLUDE_DIRS}
@@ -283,17 +286,9 @@ endforeach()
 
 set(PYTHON_EXECUTABLE ${_PYTHON_EXECUTABLE})
 
+# Depend on rosidl_generator_py generated targets from our dependencies
 foreach(_pkg_name ${rosidl_generate_interfaces_DEPENDENCY_PACKAGE_NAMES})
-  set(_pkg_install_base "${${_pkg_name}_DIR}/../../..")
-  set(_pkg_python_libname "${_pkg_name}__python")
-
-  if(WIN32)
-    target_link_libraries(${_target_name_lib} "${_pkg_install_base}/Lib/${_pkg_python_libname}.lib")
-  elseif(APPLE)
-    target_link_libraries(${_target_name_lib} "${_pkg_install_base}/lib/lib${_pkg_python_libname}.dylib")
-  else()
-    target_link_libraries(${_target_name_lib} "${_pkg_install_base}/lib/lib${_pkg_python_libname}.so")
-  endif()
+  target_link_libraries(${_target_name_lib} ${${_pkg_name}_TARGETS${rosidl_generator_py_suffix}})
 endforeach()
 
 set_lib_properties("")
@@ -305,9 +300,14 @@ if(WIN32)
 endif()
 if(NOT rosidl_generate_interfaces_SKIP_INSTALL)
   install(TARGETS ${_target_name_lib}
+    EXPORT export_${_target_name_lib}
     ARCHIVE DESTINATION lib
     LIBRARY DESTINATION lib
     RUNTIME DESTINATION bin)
+
+  # Export this target so downstream interface packages can depend on it
+  rosidl_export_typesupport_targets("${rosidl_generator_py_suffix}" "${_target_name_lib}")
+  ament_export_targets(export_${_target_name_lib})
 endif()
 
 if(BUILD_TESTING AND rosidl_generate_interfaces_ADD_LINTER_TESTS)