Renames the four relocated `PHP*` implementation classes to their public
names and removes the temporary loaders introduced by #304.

With #303 merged, Files changed contains only the class declaration
cleanup and temporary-loader removal. The resulting tree contains
neither the retired `PHP*` class names nor the `PHP/` and `loaders/`
directories.

## Stack

1. #304 — vacate the destination paths
2. #303 — move four byte-identical implementation files
3. #305 — restore the public class declarations and remove temporary
loaders

## Testing

The Data Liberation, XML, and Markdown suites pass. PHPCS and syntax
checks pass. A root-autoload check confirms the public classes load, the
retired `PHP*` classes do not, and `WP_HTML_Processor` remains a
`WP_HTML_Tag_Processor` subclass.