feat(popcount): Clean up popularity counting script

Adds the script used to generate the popularity information for all of
nixpkgs.

The README lists the (currently somewhat rough) usage instructions.

tools/nixery/group-layers/popcount

@@ -1,13 +0,0 @@
-#!/bin/bash
-set -ueo pipefail
-
-function graphsFor() {
-  local pkg="${1}"
-  local graphs=$(nix-build --timeout 2 --argstr target "${pkg}" popcount.nix || echo -n 'empty.json')
-  cat $graphs | jq -r -cM '.[] | .references[]'
-}
-
-for pkg in $(cat all-top-level.json | jq -r '.[]'); do
-  graphsFor "${pkg}" 2>/dev/null
-  echo "Printed refs for ${pkg}" >&2
-done