Set original Module to definedClass of methods when
new methods are defined to PrependedModule.

Before this commit, definedClass of method a
is class c but definedClass of method b
is PrependedModule:

```
M = Module.new
c = Class.new {
  def a; end
  prepend M
  def b; end
}
```

This commit will set definedClass of method b
to class c and fix `TestMethod#test_alias_owner`.