[FIX] resource: Adjust time span to calendar

The method `_get_work_interval` has several problems:

1) The name is not self explanatory in any way

2) It should be defined on the `resource.mixin`

3) It simply does not work (as described in its docstring).
   Here is a simplified example (only concerned about hours):
   Currently, given two attendances: 8-12 and 13-17,
   with parameters start=9 and end=18 it returns (9, 17)
   while it should return (8, 17).

   This leads to strange behaviors in the planning app:
   Given employee A with calendar 8-17 and employee B 8-16.
   Create a planning slot and assign employee A: the start and end times
   are set to 8-17.
   Then assign employee B, the start and end times are correcly set to 8-16.
   Now reassign employee A: the start and end times are not set to 8-17.

Task 2229296

closes odoo/odoo#50804

X-original-commit: 7dfab89bc0025cf6424f00b6992fef1f42463ffe
Related: odoo/enterprise#10426
Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>
Signed-off-by: lul-odoo <LucasLefevre@users.noreply.github.com>