[FIX] P3: reorder finders in meta_path

So that was a fun one: mock.patch calls would regularly fail refusing
to find the addon in odoo.addon (e.g. essentially getattr(odoo.addon,
'account_budget' deep within the bowels of mock).

Turns out the answer is that our import hooks would not be used for
many imports: while in Python 2, sys.meta_path is empty and the
default finders are run after all meta_path finders fail as noted by
the documentation[0].

However when the import system was rewritten in Python 3.3[1]
meta_path was "despecialised" and the default finders were moved to
meta_path rather than be a hidden part of the import machinery[2]:

> sys.meta_path and sys.path_hooks now store all of the meta path
> finders and path entry hooks used by import. Previously the finders
> were implicit and hidden within the C code of import instead of
> being directly exposed.

The result of this change is that ``sys.meta_path.append`` means the
default finders should take priority and the custom ones should be
fallback. This is the exact opposite of what we want.

Fix issue by ``sys.meta_path.insert``-ing our finders at the start of
the path rather than appending them at the end. This should change
nothing in Python 2 but seems to fix the issue in P3.

[0]
https://docs.python.org/2/library/sys.html?highlight=meta_path#sys.meta_path
[1] https://docs.python.org/3/whatsnew/3.3.html#importlib
[2] https://docs.python.org/3/whatsnew/3.3.html#visible-changes

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