Properly handle evaluated conditions

[mikepurvis]

Currently we handle the condition for ROS_PYTHON_VERSION via setting a distro-specific python value in the config toml:

colcon-distro/colcon_distro/config.py

Line 24 in e4f4a4a

os.environ['ROS_PYTHON_VERSION'] = str(self.distro.python_version)

However, what we don't properly handle is ROS_VERSION— this is done in a handful of chimera-type repos, where a single codebase supports both ROS 1 and 2, such as PlotJuggler:

https://github.com/facontidavide/PlotJuggler/blob/8c1ae51dfe966faa4af9c69b5284a7613e6c83f9/package.xml#L14-L22

The consequence of the current state of affairs is that when generating workspaces, Nix definitions, whatever which include a package like this, a bunch of dependencies from other-ROS show up as warnings about unresolved rosdeps.

The three main possibilities for how to better handle this are:

1. Defer evaluating the conditions. This would require support from elsewhere, in particular Defer evaluated conditions? colcon/colcon-ros#128.
2. Add an additional field to the repo_states table so that it stores PlotJuggler/8c1ae51dfe966faa4af9c69b5284a7613e6c83f9 in ROS 1 mode as a separate entity from that same version in ROS 2 mode. This could just be called ros_version, but it would be nice to figure out how to generalize it since very little else in colcon-distro is presently ROS-specific. In the extreme, this could be a whole table of potential "contexts" that the repo_states entries could key into.
3. Switch to caching the whole package.xml file (where available), and generate PackageDescriptor objects at request time— this way any contextual information about the distribution will be known.