libdeps graph linter: public deps efficiency threshold

The transitive tree of edges created by a public libdeps declaration may be added to many link lines but not actually used.

A given public libdep can be given an efficiency percentage calculated by (the number of transitive edges that really need the link)/(the number of edges in that transitive tree).

First we should implement a query so that for a given node, you can print the efficiency of all direct public libdeps on that node. 

We should then also implement a linter which checks all direct public libdeps edges efficiency percentages, and if a percentage is below a given threshold it is reported. It may be that it is desirable that a node which has, say for example a %2 percent efficiency and is actually needed in only two link lines, but is transitively added to 100 link lines, would instead just use two LIBDEPS_PRIVATE links where needed.