Allow the CommandSpec class to be resolved by the writer.

setuptools/command/easy_install.py

@@ -1988,6 +1988,8 @@ class ScriptWriter(object):
             )
     """).lstrip()
 
+    command_spec_class = CommandSpec
+
     @classmethod
     def get_script_args(cls, dist, executable=None, wininst=False):
         # for backward compatibility

setuptools/command/install_scripts.py

@@ -39,7 +39,7 @@ class install_scripts(orig.install_scripts):
             writer = ei.WindowsScriptWriter
         # resolve the writer to the environment
         writer = writer.best()
-        cmd = ei.CommandSpec.from_param(exec_param)
+        cmd = writer.command_spec_cls.from_param(exec_param)
         for args in writer.get_args(dist, cmd.as_header()):
             self.write_script(*args)