packages: Remove duplicates from package cache.

Previously the same package could appear several times if several
variables were bound to it, as is notably the case for "python"
currently.  This, in turn, would lead to obnoxious "ambiguous package
specification" messages.

* gnu/packages.scm (generate-package-cache)[expand-cache]: Change RESULT
to RESULT+SEEN and adjust accordingly.
Call 'first' on the result of 'fold-module-public-variables*'.
* tests/packages.scm ("fold-available-packages with/without cache"):
Check for lack of duplicates in FROM-CACHE.

tests/packages.scm

@@ -1037,7 +1037,8 @@
                                                       result))
                                               '()))))))
 
-    (lset= equal? no-cache from-cache)))
+    (and (equal? (delete-duplicates from-cache) from-cache)
+         (lset= equal? no-cache from-cache))))
 
 (test-assert "find-packages-by-name"
   (match (find-packages-by-name "hello")