Support getattr for prior module

[williambdean]

This ports this PR: pymc-labs/pymc-marketing#1534

It was removed from PyMC-Marketing due to the documentation build failing. However, since the
build for PyMC-extras requires specified classes and functions, it should work.

The prior class now has the convenience imports:

# Direct import of a distribution
from pymc_extras.prior import Normal, HalfNormal

distribution = Normal(mu=Normal(), sigma=HalfNormal(), dims="covariate")

# dot notation for convenience
import pymc_extras.prior as pr

distribution = pr.Normal(mu=pr.Normal(), sigma=pr.HalfNormal(), dims="covariate")

Then use like normal:

assert isinstance(distribution, pr.Prior)
assert distribution == Prior("Normal", mu=Prior("Normal"), sigma=Prior("HalfNormal"), dims="covariate")

No functional change. Just a convenience import to make things closer to the PyMC / preliz API

[williambdean]

Closes #500

[juanitorduz]

Nice 😎 !