cs: fix srcloc path conversion on load with no current-load-relative-directory

Fall back to `current-directory` when `current-load-relative-directory`
is #f.

This change also affects `deserialize` --- not because byte code
loading uses it directly in this case, but because they share a helper
function, which exposes the issue. This implementation change is
worrying (even though it makes the implementation match the
documentation), but unless we discover that some use of serialization
needs absolute paths deseialized as relative, is seems better to be
consistent everywhere about falling back to `currenrt-directory`. This
aspect of the change can be reverted separately (by adding more code)
if needed.

Closes racket/drracket#421

pkgs/racket-doc/scribblings/reference/printer.scrbl

@@ -593,14 +593,14 @@ not normally printed in a way that can be read back in, path literals
 can be written and read as part of compiled code. The
 @racket[current-write-relative-directory] parameter is used to convert
 the path to a relative path as is it written, and then
-@racket[current-load-relative-directory] parameter is used to convert
+@racket[current-load-relative-directory] parameter (falling back to
-any relative path back as it is read. The relative-path conversion
+@racket[current-directory]) is used to convert
-applies on reading whether the path was originally relative or not.
+any relative path back as it is read.
 
 For a path in a syntax object's source, if the
-@racket[current-load-relative-directory] parameter is not set of the
+@racket[current-write-relative-directory] parameter is not set or the
 path is not relative to the value of the
-@racket[current-load-relative-directory] parameter, then the path is
+@racket[current-write-relative-directory] parameter, then the path is
 coerced to a string that preserves only part of the path (an in effort
 to make it less tied to the build-time filesystem, which can be
 different than the run-time filesystem).

pkgs/racket-test-core/tests/racket/serialize.rktl

@@ -717,7 +717,7 @@
    (deserialize (serialize root #:relative-directory (build-path root "a"))))
 
   (test
-   (build-path 'same)
+   (build-path (current-directory) 'same)
    'this-dir-path
    (parameterize ([current-load-relative-directory #f])
      (deserialize (serialize (build-path root 'same) #:relative-directory root)))))

racket/collects/racket/private/relative-path.rkt

@@ -4,7 +4,8 @@
          make-path->relative-path-elements)
 
 (define (relative-path-elements->path elems)
-  (define wrt-dir (current-load-relative-directory))
+  (define wrt-dir (or (current-load-relative-directory)
+                      (current-directory)))
   (define rel-elems (for/list ([p (in-list elems)])
                       (if (bytes? p) (bytes->path-element p) p)))
   (cond

racket/src/cs/schemified/schemify.scm

@@ -48042,7 +48042,9 @@
     (lambda (e_0) (xify_0 e_0 hash2610))))
 (define relative-path-elements->path
   (lambda (elems_0)
-    (let ((wrt-dir_0 (current-load-relative-directory)))
+    (let ((wrt-dir_0
+           (let ((or-part_0 (current-load-relative-directory)))
+             (if or-part_0 or-part_0 (current-directory)))))
       (let ((rel-elems_0
              (reverse$1
               (begin