A list of comma-separated output group names, each of which optionally prefixed by a + or a -. A group prefixed by + is added to the default set of output groups, while a group prefixed by - is removed from the default set. If at least one group is not prefixed, the default set of output groups is omitted. For example, --output_groups=+foo,+bar builds the union of the default set, foo, and bar, while --output_groups=foo,bar overrides the default set such that only foo and bar are built.