Like projectConfigPackageGlobs but optional in the
sense that file globs are allowed to match nothing. The primary use
case for this is to be able to say optional-packages: */ to
automagically pick up deps that we unpack locally without erroring
when there aren't any.
As almost all commands use ProjectFlags but not all can honour
"ignore-project" flag, provide this utility to remove the flag parsing
from the help message.