Allow substitution args without ROS (#340)

Conditional import of ament_index_python for $(find ...) only

xacro/__init__.py

@@ -285,14 +285,14 @@ def eval_extension(s):
     if s == '$(cwd)':
         return os.getcwd()
     try:
-        from .substitution_args import resolve_args, ArgException, PackageNotFoundError
+        from .substitution_args import resolve_args, ArgException
         return resolve_args(s, context=substitution_args_context)
     except ImportError as e:
         raise XacroException("substitution args not supported: ", exc=e)
     except ArgException as e:
         raise XacroException("Undefined substitution argument", exc=e)
-    except PackageNotFoundError as e:
-        raise XacroException("package not found:", exc=e)
+    except Exception as e:
+        raise XacroException(f"{type(e)}: {e}", exc=e)
 
 
 class Table(dict):

xacro/substitution_args.py

@@ -41,7 +41,6 @@
 import os
 import yaml
 
-from ament_index_python.packages import get_package_share_directory, PackageNotFoundError
 from io import StringIO
 
 
@@ -138,6 +137,7 @@ def _dirname(resolved, a, args, context):
 
 
 def _eval_find(pkg):
+    from ament_index_python.packages import get_package_share_directory
     return get_package_share_directory(pkg)