Search for SYMBOL's definition of type TYPE in LIBRARY.
Visit the library in a buffer, and return a cons cell (BUFFER . POSITION),
or just (BUFFER . nil) if the definition can't be found in the file.
If TYPE is nil, look for a function definition.
Otherwise, TYPE specifies the kind of definition,
and it is interpreted via `find-function-regexp-alist'.
The search is done in the source for library LIBRARY. (fn SYMBOL TYPE LIBRARY)