Cleaning namespace for cljc files

[dotemacs]

NOTE: Still WIP, pushing early to discuss. The CI will break the build, as I didn't touch all the affected tests.

Previously it would create a namespace with a double require, from this:

(ns clean-ns.cljc-ns
  (:require [vlad.core :as vlad :refer [attr chain join present Validation]]
            #? (:cljs [goog.date.Interval :as Interval])
            [clojure.string :as string]
            [cemerick.url :as url]))

to this:

(ns clean-ns.cljc-ns
  #?@
   (:clj
    [(:require
      [cemerick.url :as url]
      [clojure.string :as string]
      [vlad.core :as vlad :refer [attr chain join present Validation]])]
    :cljs
    [(:require
      [cemerick.url :as url]
      [clojure.string :as string]
      [goog.date.Interval :as Interval]
      [vlad.core :as vlad :refer [attr chain join present Validation]])]))

Now it creates:

(ns clean-ns.cljc-ns
  (:require
  [vlad.core :as vlad :refer [attr chain join present Validation]]
  [cemerick.url :as url]
  [clojure.string :as string]
  #?(:cljs [goog.date.Interval :as interval])))

This is related to: #266

NOTE: I'll update the below after feedback.

• The commits are consistent with our contribution guidelines
• You've added tests (if possible) to cover your change(s)
• All tests are passing (run lein do clean, test)
• Code inlining with mranderson works and tests pass with inlined code (run ./build.sh install -- takes a long time)
• You've updated the changelog (if adding/changing user-visible functionality)
• You've updated the readme (if adding/changing user-visible functionality)

[vemv]

Feel free to re-take these efforts at any point