Add route for a specific Package

[jmpavlick]

This depends on https://github.com/jmpavlick/segakcap/issues/2 - we need a new Index type of

type alias Index =
    { package : { name : String, summary : String }
    , dependencies : List Package
    }

for this to work. To get there, we need to resolve the other issue first.

We need a variant for Route of ByIndex Index so that we can href to a given Elm package within our app.