Library groups are organized in trees.
Compute groups of libraries, associate root libraries with
a tree of its children. A group of libraries is defined by
the fact that these libraries have a parental relationship
and must be installed together, with the same META file.
Return the findlib root name of a group, it takes into account
containers. So the return group name is the toplevel name
for both libraries and theirs containers.
Return the root library, i.e. the first found into the group tree
that has no parent.