Allow specifying a custom PATH for which method

Closes Homebrew/homebrew#21794.

Library/Homebrew/requirements/python_dependency.rb

@@ -121,15 +121,9 @@ class PythonInstalled < Requirement
         # Note, we don't support homebrew/versions/pythonXX.rb, though.
         Formula.factory(@name).opt_prefix/"bin/python#{@min_version.major}"
       else
-        begin
         # Using the ORIGINAL_PATHS here because in superenv, the user
         # installed external Python is not visible otherwise.
-          tmp_PATH = ENV['PATH']
-          ENV['PATH'] = ORIGINAL_PATHS.join(':')
-          which(@name)
-        ensure
-          ENV['PATH'] = tmp_PATH
-        end
+        which(@name, ORIGINAL_PATHS.join(':'))
       end
     end
   end

Library/Homebrew/utils.rb

@@ -166,8 +166,8 @@ def puts_columns items, star_items=[]
   end
 end
 
-def which cmd
-  dir = ENV['PATH'].split(':').find {|p| File.executable? File.join(p, cmd)}
+def which cmd, path=ENV['PATH']
+  dir = path.split(':').find {|p| File.executable? File.join(p, cmd)}
   Pathname.new(File.join(dir, cmd)) unless dir.nil?
 end