ROS plugins: allow skip keys for rosdep

[Levi Armstrong]

It is common with developing ROS packages to have keys added to the package.xml that are not in the public rosdep. The following cases are below, but there may be more.

1. If these dependencies are internal dependencies and never get added to rosdep.

2. If a custom version of system library and we don't want to install locally. We would clone the repository into our workspace like VTK and add a package.xml with a unique package name, different than what is currently in rosdep and then use this in our packages.xml so catkin knows to build this package prior to our other packages.

Propose adding the ability to list skip keys for rosdep to solve the issue.

[Kyle Fazzari (kyrofa)]

Thanks Levi! I updated the title to reflect that this should apply to both Catkin and Colcon plugins, as both use rosdep.

[jeremie (artivis)]

The issue described here is addressed for plugins V2 in https://github.com/snapcore/snapcraft/pull/3536.

rosdep keys that are locally available are not forwarded to 'rosdep resolve' anymore and the locally available packages are used instead.

Note that it does not implement a 'skip keys' option as suggested, which shouldn't be necessary.