Import setup from setuptools instead of distutils.core (#18)

Recently users of newer distributions who build Noetic from source
noticed issues when importing setup from distutils.core.
This problem was discussed on
[Discourse](https://discourse.ros.org/t/ros-1-and-python-3-10s-deprecation-of-distutils-core/29834),
and we hope that we can make the needed updates to Noetic to allow for
future builds from source of Noetic.
As a first step, this PR introduces changes from the [Noetic Migration
Guide](https://wiki.ros.org/noetic/Migration#Setuptools_instead_of_Distutils)
that addresses the change to the setuptools module instead of
distutils.core and the corresponding buildtool_depend tags for python
2&3.

package.xml

@@ -1,3 +1,7 @@
+<?xml version="1.0"?>
+<?xml-model
+  href="http://download.ros.org/schema/package_format3.xsd"
+  schematypens="http://www.w3.org/2001/XMLSchema"?>
 <package format="3">
   <name>rqt_dep</name>
   <version>0.4.12</version>
@@ -15,6 +19,8 @@
   <author>Thibault Kruse</author>
 
   <buildtool_depend>catkin</buildtool_depend>
+  <buildtool_depend condition="$ROS_PYTHON_VERSION == 2">python-setuptools</buildtool_depend>
+  <buildtool_depend condition="$ROS_PYTHON_VERSION == 3">python3-setuptools</buildtool_depend>
 
   <exec_depend version_gte="0.2.19">python_qt_binding</exec_depend>
   <exec_depend condition="$ROS_PYTHON_VERSION == 2">python-rospkg</exec_depend>

setup.py

@@ -1,6 +1,6 @@
 #!/usr/bin/env python
 
-from distutils.core import setup
+from setuptools import setup
 from catkin_pkg.python_setup import generate_distutils_setup
 
 d = generate_distutils_setup(