Support multiple pkgnames as input and add at least one value as
requirement.

We always want to add reverse dependencies even if we already vistted
the node. A reproducer is cargo run haskell-basic-prelude and taskell
would end up not being included.

The previous approach would figure out the reverse dependencies of a
package by iterating over all packages for finding the reverse
dependencies of a package. Now we create one hashmap with the package to
reverse dependencies mapping which reduces the amount of work we have to
do and makes the program more or less constant time.