import: texlive: Load (gnu packages tex) dynamically.

Fixes a bug whereby 'guix pull' would fail with this guix-extra.drv
build failure:

  ice-9/eval.scm:163:9: ERROR:
    1. &formatted-message:
	format: "~a: patch not found\n"
	arguments: ("nettle-3.5-check-_pkcs1_sec_decrypt-msg-len.patch")

Reported by ekaitz on #guix.

Regression introduced in 3b1a12c5bf.

* guix/import/texlive.scm: Autoload (gnu packages tex).

fixlet

guix/import/texlive.scm

@@ -38,7 +38,6 @@
   #:use-module (guix upstream)
   #:use-module (guix packages)
   #:use-module (guix build-system texlive)
-  #:use-module (gnu packages tex)
   #:export (texlive->guix-package
             texlive-recursive-import))
 
@@ -111,6 +110,12 @@
                              name)))
 
 (define (tlpdb-file)
+  (define texlive-bin
+    ;; Resolve this variable lazily so that (gnu packages ...) does not end up
+    ;; in the closure of this module.
+    (module-ref (resolve-interface '(gnu packages tex))
+                'texlive-bin))
+
   (with-store store
     (run-with-store store
       (mlet* %store-monad