Support loading likelihoods from python packages

[tilmantroester]

It would be useful to support loading the build_likelihood factory from python packages, rather than file names.
For example, to support usage like firecrownIni: desc_baryon_challenge.likelihoods.firecrown_likelihood.
Or maybe better more explicitly: firecrown_likelihood_factory: desc_baryon_challenge.likelihoods.firecrown_likelihood.build_likelihood

With the option to pass build_parameters, this should allow quite generic likelihood factories that can be shipped centrally in a python package.

A specific use case is the baryon challenge: https://github.com/LSSTDESC/baryon-challenge/blob/main/src/desc_baryon_challenge/likelihoods/firecrown_likelihood.py