Starting to move macros for documenting Planet packages out of unstable.

I am leaving the bindings in unstable as they are, in case there are a few
packages out there relying on them.  My intention is to remove them after the
release of 5.1 so that developers on the trunk have a chance to find and fix
bugs in their Planet packages.

This commit: moved macro-facilitating functions from unstable/planet-syntax
to planet/syntax.

collects/planet/syntax.rkt

@@ -0,0 +1,53 @@
+#lang racket/base
+
+(provide make-planet-require-spec
+         syntax-source-planet-package
+         syntax-source-planet-package-owner
+         syntax-source-planet-package-name
+         syntax-source-planet-package-major
+         syntax-source-planet-package-minor
+         syntax-source-planet-package-symbol)
+
+(require racket/match planet/util unstable/syntax)
+
+(define (syntax-source-planet-package stx)
+  (let* ([dir (syntax-source-directory stx)])
+    (and dir (path->package-version dir))))
+
+(define (syntax-source-planet-package-owner stx)
+  (match (syntax-source-planet-package stx)
+    [(list owner name major minor) owner]
+    [_ #f]))
+
+(define (syntax-source-planet-package-name stx)
+  (match (syntax-source-planet-package stx)
+    [(list owner name major minor) name]
+    [_ #f]))
+
+(define (syntax-source-planet-package-major stx)
+  (match (syntax-source-planet-package stx)
+    [(list owner name major minor) major]
+    [_ #f]))
+
+(define (syntax-source-planet-package-minor stx)
+  (match (syntax-source-planet-package stx)
+    [(list owner name major minor) minor]
+    [_ #f]))
+
+(define (syntax-source-planet-package-symbol stx [suffix #f])
+  (match (syntax-source-planet-package stx)
+    [(list owner name major minor)
+     (string->symbol
+      (format "~a/~a:~a:~a~a"
+              owner
+              (regexp-replace "\\.plt$" name "")
+              major
+              minor
+              (if suffix (format-symbol "/~a" suffix) "")))]
+    [#f #f]))
+
+(define (make-planet-require-spec stx id/f)
+  (datum->syntax
+   stx
+   (list #'planet (syntax-source-planet-package-symbol stx id/f))
+   (or id/f stx)))