(compilation-error-list): An elt's cdr's car is again a cons (DIRECTORY .

(compilation-error-list): An elt's cdr's car is again a cons (DIRECTORY .
FILE) if it's not a marker.  The conversion to using a string containing an
expanded file name was never finished, and anyway it utterly broke
compilation-search-path and few other things.
(next-error): Expect them that way.
(compilation-parse-errors): Make them that way.
(compile-file-of-error): Function removed.
(compilation-error-filedata, compilation-error-filedata-file-name): New
defsubsts.
(compilation-next-file): Use them instead of compile-file-of-error.