model: fix module owner placebo

nitdoc still rely on the old nesting hierarchy.
But the implementation of the placebo was buggy if a nested module is
loaded before its nesting module (because a wrongful usage of
`.mmodules.first`).

This patch has two small parts

* add a method `fuzzy_owner` to a MGroup (to replace the crazy
  `.mmodules.first`)
* a special case is added when a nesting module is loaded after some of
  its nested ones

Fix #324
Signed-off-by: Jean Privat <jean@pryen.org>