Start guide on determinine if dependencies are available in ROS 2

Signed-off-by: Shane Loretz <shane.loretz@gmail.com>

Author: sloretz

source/How-To-Guides/Migrating-from-ROS1.rst

@@ -8,6 +8,7 @@ If you are new to porting between ROS 1 and ROS 2, it is recommended to read thr
    :maxdepth: 1
 
    Migrating-from-ROS1/Migrating-Packages
+   Migrating-from-ROS1/Migrating-Dependencies
    Migrating-from-ROS1/Migrating-Package-XML
    Migrating-from-ROS1/Migrating-Interfaces
    Migrating-from-ROS1/Migrating-CPP-Package-Example

source/How-To-Guides/Migrating-from-ROS1/Migrating-Dependencies.rst

@@ -0,0 +1,135 @@
+Migrating Dependencies
+======================
+
+This page shows you how to tell if all of your ROS 1 package's dependencies are available in ROS 2.
+
+.. contents:: Table of Contents
+   :depth: 2
+   :local:
+
+Prerequisites
+-------------
+
+This page assumes you have a Linux machine with :doc:`rosdep </Tutorials/Intermediate/Rosdep>` installed.
+
+Why do dependencies matter?
+---------------------------
+
+Virtually all ROS packages depend on something.
+A package that needs the transform between two points probably depends on ``tf2``.
+A package that installs URDF files probably needs ``xacro``.
+No package can work without its dependencies, so when you want to migrate any package to ROS 2 you must make sure all of its dependencies are available first.
+
+What ROS distro are you targeting?
+----------------------------------
+
+TODO this guide assumes you're targeting ROS {DISTRO}.
+
+Determine your package's dependencies
+-------------------------------------
+
+If you want to know what your package depends on, then read it's ``package.xml``.
+Every package's ``package.xml`` file must list that package's dependencies.
+
+However, there are two kinds of dependencies in ROS, and the difference is important when deciding if your package is ready to be migrated to ROS 2.
+
+* a ROS package
+* a rosdep key
+
+A ROS package is exactly what it sounds like; your ROS package may depend on one or more other ROS packages.
+A rosdep key is different.
+It describes a system dependency.
+For example, `tf2 <https://index.ros.org/p/tf2/>`__ is a ROS package, while `eigen <https://index.ros.org/d/eigen/>`__ is a rosdep key.
+Read :doc:`the tutorial on rosdep </Tutorials/Intermediate/Rosdep>` to learn more.
+
+Is it a rosdep key or a package?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+TODO use the rosdep keys command followed by the rosdep resolve command
+
+.. tabs::
+
+  .. group-tab:: Linux
+
+    .. code-block:: bash
+
+        cd /tmp
+        git clone https://github.com/ros/geometry2.git
+        rosdep keys --from-paths /tmp/geometry2/tf2_kdl | xargs rosdep resolve --os=ubuntu:focal --rosdistro=noetic
+
+
+TODO if the package to be installed starts with `ros-distro`, then it's a ROS package. Otherwise it's a ROSDep key
+
+.. tabs::
+
+  .. group-tab:: Linux
+
+    .. code-block:: bash
+
+        $ rosdep keys --from-paths /tmp/geometry2/tf2_kdl | xargs rosdep resolve --os=ubuntu:focal --rosdistro=noetic
+        #ROSDEP[eigen]
+        #apt
+        libeigen3-dev
+        #ROSDEP[cmake_modules]
+        #apt
+        ros-noetic-cmake-modules
+        #ROSDEP[tf2_ros]
+        #apt
+        ros-noetic-tf2-ros
+        #ROSDEP[catkin]
+        #apt
+        ros-noetic-catkin
+        #ROSDEP[rostest]
+        #apt
+        ros-noetic-rostest
+        #ROSDEP[liborocos-kdl-dev]
+        #apt
+        liborocos-kdl-dev
+        #ROSDEP[tf2]
+        #apt
+        ros-noetic-tf2
+        #ROSDEP[ros_environment]
+        #apt
+        ros-noetic-ros-environment
+
+
+Check if a rosdep key is available
+----------------------------------
+
+TODO It matters what OS you're using. We're deling with ssytem deps after all
+
+Check if a ROS package is available
+-----------------------------------
+
+TODO Searching ROS Index for the given ROS distro
+
+
+Has this ROS package been replaced?
+-----------------------------------
+
+Some packages haven't been migrated to ROS 2 because they were replaced with something better.
+If you can't find a package in the ROS Index, then check the table below to see if it has a replacement.
+
+TODO move_base -> nav2, ... what else?
+
+.. list-table:: Equivalent packages in ROS 1 and ROS 2
+   :widths: 25 25
+   :header-rows: 1
+
+   * - ROS 1
+     - ROS 2
+   * - catkin
+     - ament_cmake_ros
+   * - roscpp
+     - rclcpp
+   * - roslaunch
+     - launch_ros
+   * - rospy
+     - rclpy
+
+Conclusion
+----------
+
+You now know if all of your package's dependencies are available in ROS 2.
+If any dependency is not available, you must migrate it first.
+Head back to :doc:`Migrating Packages <./Migrating-Packages>` to learn how to migrate it.

source/How-To-Guides/Migrating-from-ROS1/Migrating-Packages.rst

@@ -15,7 +15,9 @@ There are two different kinds of package migrations:
 Prerequisites
 -------------
 
-Before being able to migrate a ROS 1 package to ROS 2 all of its dependencies must be available in ROS 2.
+Virtually all ROS packages depend on at least one other package, and no package can be migrated to ROS 2 until its dependencies are available.
+Are all of your package's dependencies available in ROS 2?
+Read the :doc:`Migrating Dependencies guide <./Migrating-Dependencies>` to learn how to figure that out.
 
 Package.xml format version
 --------------------------