read classes from file when lexing

[Qubus0]

as requested by @mhilbrunner in #11217 (comment) we can read the class reference index and grab all the file names, making the lexer exactly as up-to-date as the class reference.

• this does read pretty much all class files once because the class names in the index are not capitalised, which isn't quite optimal
• no web requests at least

the full repo is available during pipeline run, right? so the files should be readable