Help find catkin python for install underlays

This is a follow on from ros#397

cmake/templates/generate_cached_setup.py.in

@@ -4,10 +4,19 @@ import os
 import stat
 import sys
 
-# find the import relatively if available to work before installing catkin or overlaying installed version
+# find the import for catkin's python package - either from source space or from an installed underlay
 if os.path.exists(os.path.join('@catkin_EXTRAS_DIR@', 'catkinConfig.cmake.in')):
     sys.path.insert(0, os.path.join('@catkin_EXTRAS_DIR@', '..', 'python'))
-from catkin.environment_cache import generate_environment_script
+try:
+    from catkin.environment_cache import generate_environment_script
+except ImportError:
+    # search for catkin package in all workspaces and prepend to path
+    for workspace in "@CATKIN_WORKSPACES@".split(';'):
+        python_path = os.path.join(workspace, '@CATKIN_GLOBAL_PYTHON_DESTINATION@')
+        if os.path.isdir(os.path.join(python_path, 'catkin')):
+            sys.path.insert(0, python_path)
+            break
+    from catkin.environment_cache import generate_environment_script
 
 code = generate_environment_script('@CATKIN_DEVEL_PREFIX@/env.@script_ext@')