Remove read access on `ir.model`, `ir.model.fields`, `ir.model.fields.selection` and `ir.model.data` for employees.
To access such records, the code must use the helpers (`_get`, `ref`,...) instead of CRUD operations directly.
Simplify the helpers to ir.model.data that were public and redundant.
self.env.ref is encouraged instead but with the slight drawback that `ref` makes a call to `exists()` (`_xmlid_lookup` can be used if this wants to be avoided).
Part of task-id 8203
closesodoo/odoo#69120
Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>