Fix cookbook scraper bugs and re-run (#697)

* fix path separator bug in cookbook scraper

* update lockfiles

* add category to image name to avoid conflicts

e.g.
testing/integration/introduction
testing/widget/introduction

* fix form_app symlink

* remove webimages directory

* remove vision_challenge

vision_challenge doesn't build on the dev channel

* update pubspec.yaml in _tool and form_app

web/_tool/pubspec.lock

@@ -14,27 +14,27 @@ packages:
       name: charcode
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.1.3"
+    version: "1.2.0-nullsafety.3"
   markdown:
     dependency: "direct main"
     description:
       name: markdown
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.1.8"
+    version: "4.0.0-nullsafety.0"
   meta:
     dependency: transitive
     description:
       name: meta
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.2.4"
+    version: "1.3.0-nullsafety.6"
   path:
     dependency: "direct main"
     description:
       name: path
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.7.0"
+    version: "1.8.0-nullsafety.3"
 sdks:
-  dart: ">=2.6.0 <3.0.0"
+  dart: ">=2.12.0-0 <3.0.0"

web/_tool/pubspec.yaml

@@ -1,6 +1,9 @@
 name: tool
 publish_to: none
 
+environment:                                                            
+  sdk: '>=2.10.0 <3.0.0'
+
 dependencies:
-  markdown: ^2.0.3
-  path: ^1.6.2
+  markdown: ^4.0.0-nullsafety.0
+  path: ^1.8.0-nullsafety.3