[IMP] core: improve python external dependencies check

Use pkg_resources.get_distribution() to check if python external
dependencies are installed, instead of trying an import.
The import test is preserved as a backwards compatibility measure.

This better expresses which python distribution needs to be installed
from PyPI and allows declaring a minimum supported version using PEP 440
version specifiers.

Closes #25541

closes odoo/odoo#25549

Signed-off-by: Christophe Simonis <chs@odoo.com>