Case of modifier class names

[matthewfeickert]

These (modifier builder and combine classes added in PR #1625) are all classes but we have them named in snake_case. This could be confusing to people in the API so might want to consider later on renaming them to CamelCase.

Originally posted by @matthewfeickert in https://github.com/scikit-hep/pyhf/pull/1625#discussion_r729335756

[matthewfeickert]

Labeling a v0.7.0 as if we're going to break APIs might as well do as much at once.