Friendship between modules

Such relationship between modules, compared to parent-child relationship, is more social. These two modules can be many-to-many mapping, e.g. An A can have many Bs while a B can have many A's.

API examples

    my ($group1, $group)=map{Group->new(-name=>$_)} qw(group1 group2);
    
    my $person=Person->new(
        -groups => [$group1, $group2]
    );

    my $person=Person->new(
        -groups=>[
            [$group1, {-rank=>1}],
            [$group2, {-rank=>10}]
        ]
    );

    my $person=Person->new(
        -groups=>{
            $group1 => {-rank=>1},
            $group2 => {-rank=>10}
        }
    );