Add logic for excluding group workaround dependencies

[cottsay]

This represents the first step in the journey to proper support for group dependencies in ros_buildfarm. Specifically, it sets the mechanism by which we disable the existing "pre-resolved" group dependencies present in the package manifests in the source repositories.

I chose the name DISABLE_GROUPS_WORKAROUND to act as the "feature flag" for a tool to use when expressing that it has support for group dependencies and doesn't want the pre-resolved dependencies to be present. I'm open to suggestions for a better name.

An example of how the package manifests would be modified for this change:

-  <build_depend>rmw_connextdds</build_depend>
+  <build_depend condition="$DISABLE_GROUPS_WORKAROUND != 1">rmw_connextdds</build_depend>

All manifests which pre-resolve groups would need to change, which (I believe) includes all manifests containing a <group_depend> element.

At present, build_file.include_group_dependencies is never True in production, so this flag should never be activated and no change in behavior is expected to any supported platforms or distributions.

[nuclearsandwich]

DISABLE_GROUPS_WORKAROUND

I don't think the conditional value needs to change otherwise the only reasonable value would grow long enough to be worthy of a Fiona Apple album title.

But a thing I think could be helpful: have a tracking issue that gives a scanty summary of the status quo, how we're planning to change it, and open to track progress on that would also give us a url to link in an XML comment explaining what this is for (and thus helping people decide when to use it).

If instead this is "buildfarm only" then using BUILDFARM in the condition name seems reasonable.
Also, we could consider adding the issue number as an ID to the string on the very remote chance that we have multiple GROUPS_WORKAROUNDs that we need to disable over time.

[cottsay]

a tracking issue that gives a scanty summary of the status quo, how we're planning to change it, and open to track progress on that would also give us a url to link in an XML comment explaining what this is for

Done: ros-infrastructure/catkin_pkg#369