Unify `distributions/transform.py` and `logprob/transform.py`

Description

These modules and, more importantly, respective tests are quite redundant. This is due to part of the functionality being moved to aeppl and then back to pymc via the logprob submodule in the past.

We should probably get rid of distributions/transforms, and possibly split the latter in two files: one for ValueTransforms and RVTransform classes and subclasses, and the other for find_measurable_transform rewrites and related functionality, as that module is getting pretty large.