[IMP] hr_holidays: perf improvement

Previously, a performance improvement landed on 5a2efd2e.
It totally makes sense, but there's a corner case that's actually hurt by that
commit: there are cases where the heavy computation of employee_quantity_available
is simply not used by the following loop.

This commit solves it.

It also improves the readability of the code by unpacking and properly naming
some variables.

closes odoo/odoo#123051

X-original-commit: 902305908780336050a0d5b778727a6c06c7a2fd
Signed-off-by: Kevin Baptiste <kba@odoo.com>