fix(manager/poetry): prioritize datasource-scoped credentials when...

fix(manager/poetry): prioritize datasource-scoped credentials when authenticating with poetry sources (#14255)

* feat: use private gitlab pypi registry together with gitlab host

* docs: slim down documentation a bit

* fix: correctly normalize source name

* fix: correctly handle fallback

* refactor: apply review suggestions

* Update docs/usage/getting-started/private-packages.md

Co-authored-by: HonkingGoose <34918129+HonkingGoose@users.noreply.github.com>

* Update docs/usage/getting-started/private-packages.md

Co-authored-by: HonkingGoose <34918129+HonkingGoose@users.noreply.github.com>

Co-authored-by: Marion Deveaud <marion.deveaud@siemens.com>
Co-authored-by: HonkingGoose <34918129+HonkingGoose@users.noreply.github.com>