cmake: Substantial cleanups and simplifications

We now have a cmake-format config file. We no longer use list variables for sources, instead using target_sources when we need to add, in accordance with current best practice. (This makes it a lot easier to edit too: sources and libraries stay next to each other.) There's no more include_directories(), add_definitions(), or other gently-deprecated directory-scoped commands, nor any CMake scripts that include a parent directory reference (named targets instead).

Mostly passes a cmake-lint too (sibling tool to cmake-format)