[FIX] tools: no pollution during export translation

Before this commit, the export of translations was incorrect for code
and model translations:

For code translations, the field 'name' was used for the matching
while the _() method explicitly use None in the _get_source call to
only use the field 'src' for the search.

For model translations, the field 'res_id' was not used when searching
for a translation.
For instance, if a ir.model.fields did not have translated label,
exporting the translations was using the translations of the first
field having the same source.
While this could be convient during the import (to be discussed),
doing so in an export of translation is clearly an unexpected
side-effect.

closes odoo/odoo#40879

Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>