Return a pair `(BUFFER . POINT)' pointing to the definition of VARIABLE.
Finds the library containing the definition of VARIABLE in a buffer and
the point of the definition. The buffer is not selected.
If the variable's definition can't be found in the buffer, return (BUFFER).
The library where VARIABLE is defined is searched for in FILE or
`find-function-source-path', if non-nil, otherwise in `load-path'.
(fn VARIABLE &optional FILE)