emacs: Speed up starting the REPL.

Delay initializing variables.

* emacs/guix-main.scm: (%packages): Rename to...
(%package-vhash): ... this.  Delay setting the value.
(package-vhash): New procedure (wrapper for '%package-vhash').
(%package-table): Delay setting the value.
(package-table): New procedure (wrapper for '%package-table').
(package-by-address, packages-by-name+version): Use wrappers.

emacs/guix-main.scm

@@ -103,24 +103,34 @@ return two values: name and version.  For example, for SPEC
 (define name+version->key cons)
 (define key->name+version car+cdr)
 
-(define %packages
-  (fold-packages (lambda (pkg res)
-                   (vhash-consq (object-address pkg) pkg res))
-                 vlist-null))
+(define %package-vhash
+  (delay
+    (fold-packages (lambda (pkg res)
+                     (vhash-consq (object-address pkg) pkg res))
+                   vlist-null)))
+
+(define (package-vhash)
+  "Return vhash of 'package ID (address)'/'package' pairs."
+  (force %package-vhash))
 
 (define %package-table
-  (let ((table (make-hash-table (vlist-length %packages))))
-    (vlist-for-each
-     (lambda (elem)
-       (match elem
-         ((address . pkg)
-          (let* ((key (name+version->key (package-name pkg)
-                                         (package-version pkg)))
-                 (ref (hash-ref table key)))
-            (hash-set! table key
-                       (if ref (cons pkg ref) (list pkg)))))))
-     %packages)
-    table))
+  (delay
+    (let ((table (make-hash-table (vlist-length (package-vhash)))))
+      (vlist-for-each
+       (lambda (elem)
+         (match elem
+           ((address . pkg)
+            (let* ((key (name+version->key (package-name pkg)
+                                           (package-version pkg)))
+                   (ref (hash-ref table key)))
+              (hash-set! table key
+                         (if ref (cons pkg ref) (list pkg)))))))
+       (package-vhash))
+      table)))
+
+(define (package-table)
+  "Return hash table of 'name+version key'/'list of packages' pairs."
+  (force %package-table))
 
 (define (manifest-entry->name+version+output entry)
   (values
@@ -330,12 +340,12 @@ Example:
 ;;; Finding packages.
 
 (define (package-by-address address)
-  (match (vhash-assq address %packages)
+  (match (vhash-assq address (package-vhash))
     ((_ . package) package)
     (_ #f)))
 
 (define (packages-by-name+version name version)
-  (or (hash-ref %package-table
+  (or (hash-ref (package-table)
                 (name+version->key name version))
       '()))