Removes requires if they are not referenced in the ns anymore

- simple require
  [clojure.string] looks for clojure.string/
- :as keyword
  [clojure.string :as s] looks for s/ references
- :refer keyword
  [clojure.string :refer [split join]] looks for fn-s in the refer list
- prefix list optionally combined with :as and :refer
  [clojure string [set :as st :refer [difference union]]]
  looks for clojure.string/, set/ and difference, union fn-s
- keeps require flags if they are present

- not handling :refer :all (can't really do without repl or jvm)
  but keeps them intact
- does not keep the whitespaces, can be considered as a feature
  but defo needs improvement for more complex prefix list statements

- want to proceed with import same way