Add setuptools dependency back in. (#141)

It turns out that non-ament_python packages (like most
ament packages) were taking an implicit dependency on setuptools
via this package.  When we removed this in commit
8325581, we removed that
implicit dependency and hence broke the packages.

The correct long-term solution here is probably to make
those packages explicitly depend on setuptools.  But for
a short-term fix, we can restore the dependency here as
a buildtool_export_depend.

Signed-off-by: Chris Lalancette <[email protected]>

package.xml

@@ -13,6 +13,8 @@
   <author email="[email protected]">Dirk Thomas</author>
   <author email="[email protected]">Mabel Zhang</author>
 
+  <buildtool_export_depend>python3-setuptools</buildtool_export_depend>
+
   <exec_depend>python3-importlib-metadata</exec_depend>
   <exec_depend>python3-importlib-resources</exec_depend>