language/python: add package_available? helper

Add helper to check whether a module is importable.

Closes Homebrew/homebrew#37345.

Signed-off-by: Tim D. Smith <git@tim-smith.us>

Library/Homebrew/language/python.rb

@@ -92,5 +92,9 @@ module Language
         --record=installed.txt
       ]
     end
+
+    def self.package_available? python, module_name
+      quiet_system python, "-c", "import #{module_name}"
+    end
   end
 end