pkg/lib: change pkg-stage contract to return set

Before: contract expected list but the function returns a set
After: contract expects a set

fix #3029

pkgs/racket-doc/pkg/scribblings/lib.scrbl

@@ -3,6 +3,7 @@
           (for-label (except-in racket/base
                                 remove)
                      racket/contract/base
+                     (only-in racket/set set/c)
                      pkg
                      pkg/lib
                      (only-in pkg/db current-pkg-catalog-file)
@@ -189,7 +190,7 @@ is a directory within @racket[path]).
                     [#:force-strip? force-strip? boolean? #f]
                     [#:use-cache? use-cache? boolean? #f]
                     [#:quiet? quiet? boolean? #t])
-         (values string? path? (or/c #f string?) boolean? (listof module-path?))]{
+         (values string? path? (or/c #f string?) boolean? (set/c module-path?))]{
 
 Locates the implementation of the package specified by @racket[desc]
 and downloads and unpacks it to a temporary directory (as needed).

racket/collects/pkg/lib.rkt

@@ -1,5 +1,6 @@
 #lang racket/base
 (require racket/contract/base
+         (only-in racket/set set/c)
          net/url
          "path.rkt"
          "private/desc.rkt"
@@ -232,7 +233,7 @@
                           path?
                           (or/c #f string?)
                           boolean?
-                          (listof module-path?)))]
+                          (set/c module-path? #:cmp 'equal #:kind 'immutable)))]
   [pkg-config-catalogs
    (-> (listof string?))]
   [pkg-catalog-update-local