This issue captures a few minor aspects of the placements API which need improving. Namely:

1. We're not checking whether the plugintype (and, by extension, the plugin) is deprecated when loading the placement types into core during upgrade. In fact, I accidentally used a deprecated component in the test and it was still passing!
   • Cause: Not all methods in core_component are restricted during plugin deprecation, and we're using one of the ones that WILL return deprecated plugins (\core\component::get_component_directory).
   • Solution: Add a check and skip deprecated plugins during this loading process + test coverage.
2. We're not checking the component portion of the placement type string matches a real component in placements_manager::is_valid_placement_type_string.
   • Cause: we use a simple regex right now.
   • Solution: Let's preg split this and validate the component portion properly + test coverage.
   • Bonus: Let's validate that the owning component (the component advertising the placement type) matches the prefix.