Refactor backend classes to match pre/postflight

From legacy forms "after_install", etc.
* abstract out `method_missing_message` to Cask::Utils
* provide `method_missing` coverage in pre/postflight blocks (fixes #7445)
* fix old-style rendering of Cask name as a classname in
  `method_missing` messages
Merge pull request #7642 from rolandwalker/refactor_block_names

Refactor backend classes to match preflight/postflight