backport flat_map for Ruby 1.8

Code is copied from
https://github.com/marcandre/backports/blob/master/lib/backports/1.9.2/enumerable/flat_map.rb
(MIT License by Marc-Andre Lafortune)

Closes Homebrew/homebrew#42543.

Signed-off-by: Xu Cheng <xucheng@me.com>

Library/Homebrew/extend/enumerable.rb

@@ -0,0 +1,11 @@
+module Enumerable
+  def flat_map
+    return to_enum(:flat_map) unless block_given?
+    r = []
+    each do |*args|
+      result = yield(*args)
+      result.respond_to?(:to_ary) ? r.concat(result) : r.push(result)
+    end
+    r
+  end unless method_defined?(:flat_map)
+end

Library/Homebrew/global.rb

@@ -3,6 +3,7 @@ require "extend/fileutils"
 require "extend/pathname"
 require "extend/ARGV"
 require "extend/string"
+require "extend/enumerable"
 require "os"
 require "utils"
 require "exceptions"