'net' with deepest occurrence of each pkg only

gistfile1.txt

If you take the full `net` tree, for each package find the deepest
level of tree where it appears, and delete other imports of that 
package that are nearer the root, you get something like:

net
    os
        io
            errors
        time
            syscall
                sync
                    runtime
                    sync/atomic
    math/rand
        math
    sort

Yay: preserves the depth, still a tree so no tangle of arrows. 
Boo: hides most of the relationships, like net imports sync.

If same pkg's imported twice at same depth, you're stuck picking arbitrarily
which import to show.