utils: 'current-source-directory' resolves relative file names at run time.

* guix/utils.scm (absolute-dirname): New procedure.
(current-source-directory): Emit code to use it instead of calling
'search-path'.

guix/utils.scm

@@ -702,6 +702,18 @@ output port, and PROC's result is returned."
 ;;; Source location.
 ;;;
 
+(define (absolute-dirname file)
+  "Return the absolute name of the directory containing FILE, or #f upon
+failure."
+  (match (search-path %load-path file)
+    (#f #f)
+    ((? string? file)
+     ;; If there are relative names in %LOAD-PATH, FILE can be relative and
+     ;; needs to be canonicalized.
+     (if (string-prefix? "/" file)
+         (dirname file)
+         (canonicalize-path (dirname file))))))
+
 (define-syntax current-source-directory
   (lambda (s)
     "Return the absolute name of the current directory, or #f if it could not
@@ -711,11 +723,13 @@ be determined."
        (match (assq 'filename (syntax-source s))
          (('filename . (? string? file-name))
           ;; If %FILE-PORT-NAME-CANONICALIZATION is 'relative, then FILE-NAME
-          ;; can be relative.  In that case, we try to find out the absolute
+          ;; can be relative.  In that case, we try to find out at run time
-          ;; file name by looking at %LOAD-PATH.
+          ;; the absolute file name by looking at %LOAD-PATH; doing this at
+          ;; run time rather than expansion time is necessary to allow files
+          ;; to be moved on the file system.
           (if (string-prefix? "/" file-name)
               (dirname file-name)
-              (and=> (search-path %load-path file-name) dirname)))
+              #`(absolute-dirname #,file-name)))
          (_
           #f))))))