[pp-1] Deprecate drupal_verify_install_file

Working on this.

I realize this will conflict with 📌 [pp-3] Split up and refactor system_requirements() into OOP hooks Active is therefore postponed on that, but wanted to make a start to see what changes are needed.

Great first pass! It's not used all that much, looks like system.install, install.core.inc and a couple of tests. Beyond that there are two spelling failures.