Fix process running during package creation (#14508)

There were some problems I introduced with the last PR for this. It passed the test, but failed in practice.

This adds tests for those failure cases, adds a "--help" and fixes the test so that it doesn't try and actually download MinGit as part of the Windows test.

I added package:platform as a dependency, so I did a force upgrade on the packages.

Also, re-enabling 'create package' in the cache warming code, now that #14448 is fixed.

examples/flutter_gallery/pubspec.yaml

diff --git a/examples/flutter_gallery/pubspec.yaml b/examples/flutter_gallery/pubspec.yaml
index ca2eb9c..9aed97a 100644
--- a/examples/flutter_gallery/pubspec.yaml
+++ b/examples/flutter_gallery/pubspec.yaml
@@ -9,7 +9,7 @@
   string_scanner: 1.0.2
   url_launcher: 2.0.1
   cupertino_icons: 0.1.1
-  video_player: 0.1.1
+  video_player: 0.2.0
 
   # Also update dev/benchmarks/complex_layout/pubspec.yaml
   flutter_gallery_assets:
@@ -23,7 +23,7 @@
   flutter_driver:
     sdk: flutter
 
-  args: 1.2.0 # TRANSITIVE DEPENDENCY
+  args: 1.3.0 # TRANSITIVE DEPENDENCY
   async: 2.0.3 # TRANSITIVE DEPENDENCY
   barback: 0.15.2+14 # TRANSITIVE DEPENDENCY
   boolean_selector: 1.0.2 # TRANSITIVE DEPENDENCY
@@ -38,7 +38,7 @@
   http: 0.11.3+16 # TRANSITIVE DEPENDENCY
   http_multi_server: 2.0.4 # TRANSITIVE DEPENDENCY
   http_parser: 3.1.1 # TRANSITIVE DEPENDENCY
-  io: 0.3.1 # TRANSITIVE DEPENDENCY
+  io: 0.3.2+1 # TRANSITIVE DEPENDENCY
   isolate: 1.1.0 # TRANSITIVE DEPENDENCY
   js: 0.6.1 # TRANSITIVE DEPENDENCY
   json_rpc_2: 2.0.7 # TRANSITIVE DEPENDENCY