Moved this-package-version-symbol from unstable/planet to planet/version.

Could not put it in planet/util because it depends on planet/util at phase 1.
Re-exported the other "this-package" macros from planet/version.
Modified this-package-in to take multiple suffix identifiers and to treat the
suffix "main" specially, and generate (planet owner/name:X:Y) instead of
(planet owner/name:X:Y/main).

collects/planet/version.rkt

@@ -0,0 +1,26 @@
+#lang racket/base
+
+(provide this-package-version
+         this-package-version-name
+         this-package-version-owner
+         this-package-version-maj
+         this-package-version-min
+         this-package-version-symbol
+         this-package-in)
+
+(require
+  planet/util
+  (for-syntax
+    racket/base
+    racket/require-transform
+    syntax/parse
+    planet/syntax))
+
+(define-syntax this-package-in
+  (make-require-transformer
+    (syntax-parser
+      [(~and src (_ (~or (~datum main) suffix:id) ...))
+       (expand-import
+         #`(combine-in
+             #,@(for/list ([suf (in-list (attribute suffix))])
+                  (make-planet-require-spec #'src suf))))])))