[FIX] models: "ORDER BY" on many2one fields

When ordering results on a many2one fields, results are ordered by
order of the target model. The code was wrongly assuming that this
`_order` attribute only contains `_classic_read` fields (that can be
directly read from the table in database). Now correctly generate the
"ORDER BY" clause using the current table alias.

`res.users` can now be sorted by name.