flatten only top level to avoid breaking values that are collections

[loomis]

The function map-as-dep flattens values that are collections (like :exclusions), creating an invalid specification. The following shows this behavior:

(set-env!
 :dependencies '[[org.clojure/clojure "1.8.0"]])

(require '[clojure.test :refer [is]])

(def dep-map {:project 'myproject
              :version "1.0.0"
              :exclusions '[excl1 excl2]})

(def dep '[myproject "1.0.0" :exclusions [excl1 excl2]])

(is (= dep (boot.util/map-as-dep dep-map)))

which results in the output:

FAIL in () (boot.user4426527561266792223.clj:23)
expected: (= dep (boot.util/map-as-dep dep-map))
  actual: (not (= [myproject "1.0.0" :exclusions [excl1 excl2]] [myproject "1.0.0" :exclusions excl1 excl2]))

This PR flattens only the top-level, preserving the values that are collections.

Let me know if there's a place to add unit tests. If so, I can add a test of this behavior.

[micha]

Looks great, thanks! There are tests in boot/pod/test/ (relative to repo root) if you would like to add a unit test 👍