fix problem with path->collects-relative

An internal error could be triggered by a path immediately
inside the "pkgs" directory.

pkgs/racket-pkgs/racket-test/tests/pkg/path.rkt

@@ -1,5 +1,6 @@
 #lang racket/base
-(require pkg/path)
+(require pkg/path
+         setup/dirs)
 
 (module+ test
   (require rackunit)
@@ -18,6 +19,9 @@
   (check-equal? (path->pkg (find-system-path 'temp-dir))
                 #f)
 
+  (check-equal? (path->pkg (build-path (find-pkgs-dir) "pkgs.rktd"))
+                #f)
+
   (check-equal? (call-with-values
                     (lambda () (path->pkg+subpath+collect (collection-file-path "serve-catalog.rkt" "distro-build")))
                   list)

racket/collects/pkg/path.rkt

@@ -111,13 +111,15 @@
       ;; name followed by "+<n>")
       (define len (length d))
       (define pkg-name (path-element->string (list-ref p len)))
-      (values (if (regexp-match? #rx"[+]" pkg-name) ; +<n> used as an alternate path, sometimes
-                  (regexp-replace #rx"[+].*$" pkg-name "")
-                  pkg-name)
-              (build-path* (list-tail p (add1 len)))
-              (and want-collect?
-                   (let ([i (hash-ref (read-pkg-db/cached) pkg-name #f)])
-                     (and i (sc-pkg-info? i) (sc-pkg-info-collect i)))))]
+      (if (regexp-match? #rx"pkgs[.]rktd" pkg-name)
+          (values #f #f #f) ; don't count the database as a package
+          (values (if (regexp-match? #rx"[+]" pkg-name) ; +<n> used as an alternate path, sometimes
+                      (regexp-replace #rx"[+].*$" pkg-name "")
+                      pkg-name)
+                  (build-path* (list-tail p (add1 len)))
+                  (and want-collect?
+                       (let ([i (hash-ref (read-pkg-db/cached) pkg-name #f)])
+                         (and i (sc-pkg-info? i) (sc-pkg-info-collect i))))))]
      [else
       ;; Maybe it's a linked package
       (define pkgs-dir (get-pkgs-dir scope))

racket/collects/setup/collects.rkt

@@ -21,7 +21,7 @@
     (define-values (pkg subpath pkg-collect) 
       (path->pkg+subpath+collect simple-p #:cache cache))
     (cond
-     [pkg
+     [(and pkg (not (eq? 'same subpath)))
       (define p-l (map path-element->string (reverse (explode-path subpath))))
       (define new-c-l (let ([l (reverse (cdr p-l))])
                         (if pkg-collect