Yang parser should support detect grouping name collisions

All grouping names defined within a parent node or at the top
level of the module or its submodules share the same grouping
identifier namespace. This namespace is scoped to all descendant
nodes of the parent node or module. This means that any
descendant node may use that grouping, and it MUST NOT define a
grouping with the same name.

module test {
    grouping grp;
    container cont

}