pmb.parse: Respect provider_priority if multiple providers exist

See the detailed commit message with an explanation and example use case.

To test, trust the added test case or checkout https://gitlab.com/Minecrell/pmaports/-/commits/paella-db410c-firmware and verify that firmware-qcom-db410c-wcnss gets installed for bq-paella during pmbootstrap install instead of firmware-bq-picmt-wcnss.