gexp: 'gexp->file' emits code to set '%load-path'.

* guix/gexp.scm (gexp->file): Add #:set-load-path? parameter and honor it.
* gnu/system.scm (operating-system-parameters-file): Pass
  #:set-load-path? #f.
* doc/guix.texi (G-Expressions): Adjust accordingly.

doc/guix.texi

@@ -3943,8 +3943,12 @@ script, and @var{modules} is the list of modules visible to that script.
 This is the declarative counterpart of @code{gexp->script}.
 @end deffn
 
-@deffn {Monadic Procedure} gexp->file @var{name} @var{exp}
+@deffn {Monadic Procedure} gexp->file @var{name} @var{exp} @
+            [#:set-load-path? #t]
 Return a derivation that builds a file @var{name} containing @var{exp}.
+When @var{set-load-path?} is true, emit code in the resulting file to
+set @code{%load-path} and @code{%load-compiled-path} to honor
+@var{exp}'s imported modules.
 
 The resulting file holds references to all the dependencies of @var{exp}
 or a subset thereof.

gnu/system.scm

@@ -731,7 +731,8 @@ this file is the reconstruction of GRUB menu entries for old configurations."
                                    (kernel #$(operating-system-kernel os))
                                    (kernel-arguments
                                     #$(operating-system-kernel-arguments os))
-                                   (initrd #$initrd)))))
+                                   (initrd #$initrd))
+                #:set-load-path? #f)))
 
 
 ;;;

guix/gexp.scm

@@ -1026,15 +1026,29 @@ its search path."
                            (write '(ungexp exp) port)
                            (chmod port #o555)))))))
 
-(define (gexp->file name exp)
+(define* (gexp->file name exp #:key (set-load-path? #t))
-  "Return a derivation that builds a file NAME containing EXP."
+  "Return a derivation that builds a file NAME containing EXP.  When
-  (gexp->derivation name
+SET-LOAD-PATH? is true, emit code in the resulting file to set '%load-path'
-                    (gexp
+and '%load-compiled-path' to honor EXP's imported modules."
-                     (call-with-output-file (ungexp output)
+  (match (if set-load-path? (gexp-modules exp) '())
-                       (lambda (port)
+    (()                                           ;zero modules
-                         (write '(ungexp exp) port))))
+     (gexp->derivation name
-                    #:local-build? #t
+                       (gexp
-                    #:substitutable? #f))
+                        (call-with-output-file (ungexp output)
+                          (lambda (port)
+                            (write '(ungexp exp) port))))
+                       #:local-build? #t
+                       #:substitutable? #f))
+    ((modules ...)
+     (mlet %store-monad ((set-load-path (load-path-expression modules)))
+       (gexp->derivation name
+                         (gexp
+                          (call-with-output-file (ungexp output)
+                            (lambda (port)
+                              (write '(ungexp set-load-path) port)
+                              (write '(ungexp exp) port))))
+                         #:local-build? #t
+                         #:substitutable? #f)))))
 
 (define* (text-file* name #:rest text)
   "Return as a monadic value a derivation that builds a text file containing