raco pkg show: add -v'/--version' argument for selecting a Racket version

This is sortof a step toward `raco pkg migrate'.

collects/planet2/lib.rkt

@@ -31,6 +31,8 @@
   (make-parameter #f))
 (define current-install-version-specific?
   (make-parameter #t))
+(define current-show-version
+  (make-parameter (version)))
 (define current-pkg-error
   (make-parameter (lambda args (apply error 'pkg args))))
 
@@ -101,7 +103,7 @@
   (build-path (cond
                [(current-install-system-wide?) (find-lib-dir)]
                [(current-install-version-specific?)
-                (build-path (find-system-path 'addon-dir) (version))]
+                (build-path (find-system-path 'addon-dir) (current-show-version))]
                [else
                 (find-system-path 'addon-dir)])
               "pkgs"))
@@ -1124,6 +1126,8 @@
    (parameter/c boolean?)]
   [current-install-version-specific?
    (parameter/c boolean?)]
+  [current-show-version
+   (parameter/c string?)]
   [current-pkg-error 
    (parameter/c procedure?)]
   [pkg-desc 

collects/planet2/main.rkt

@@ -1,5 +1,6 @@
 #lang racket/base
-(require racket/function
+(require (only-in racket/base [version r:version])
+         racket/function
          raco/command-name
          "lib.rkt"
          "commands.rkt"
@@ -113,12 +114,13 @@
   [#:bool installation ("-i") "Show only the installation-wide package database"]
   [#:bool shared ("-s") "Show only the user-specific all-version package database"]
   [#:bool user ("-u") "Show only the user- and version-specific package database"]
+  [(#:str #f) version ("-v") "Show only user--specific packages for specified version"]
   #:args ()
   (define only-mode (cond
                      [installation 'i]
                      [shared 's]
                      [user 'u]
-                     [else #f]))
+                     [else (if version 'u #f)]))
   (for ([mode '(i s u)])
     (when (or (eq? mode only-mode) (not only-mode))
       (unless only-mode
@@ -128,7 +130,8 @@
                          [(u) "User-specific, version-specific:"])))
       (parameterize ([current-install-system-wide? (eq? mode 'i)]
                      [current-install-version-specific? (eq? mode 'u)]
-                 [current-pkg-error (pkg-error 'show)])
+                     [current-pkg-error (pkg-error 'show)]
+                     [current-show-version (or version (r:version))])
         (with-package-lock
          (show-cmd (if only-mode "" " "))))))]
  [config

collects/planet2/scribblings/planet2.scrbl

@@ -291,6 +291,7 @@ listed, this command fails atomically. It accepts the following @nonterm{option}
  @item{@DFlag{installation} or @Flag{i} --- Show only installation-wide packages.}
  @item{@DFlag{user} or @Flag{u} --- Show only user-specific, version-specific packages.}
  @item{@DFlag{shared} or @Flag{s} --- Show only user-specific, all-version packages.}
+ @item{@DFlag{version} @nonterm{vers} or @Flag{v} @nonterm{vers} --- Show only user-specific packages for Racket version @nonterm{vers}.}
  ]
 }