portage.sets.shell.CommandOutputSet
As the name says, this class creates a package set based on the output of
a given command. The command is run once when the set is accessed
for the first time during the current session.
In single set configurations this class supports the following options:
command: Required. Specifies the command
that should be executed to generate the package set. It should
output a newline separated list of simple and/or versioned atoms
on stdout.