Rationalize Dart mojo and sky package structure

mojo/public/dart/README.md

 Dart Mojo Applications
 ====
 
 ## Mojo Application API
 
 *TODO(zra)*
 
 ## Application Packaging
 
 All Dart sources for a Mojo application are collected in a specially formatted
 zip file, which is understood by Dart's content handler in the Mojo shell.
 This section describes what the various parts of that package are, and how they
 all make it to the right place.
 
 ### GN Template
 
 Dart Mojo applications are built with the GN template
-'dart_packaged_application' defined in `//mojo/public/dart/rules.gni`.
+'dartzip_packaged_application' defined in `//mojo/public/dart/rules.gni`.
 Here is an example:
 
 
 ```
-dart_packaged_application("foo") {
+dartzip_packaged_application("foo") {
   output_name = "dart_foo"
   uses_pub = true
   sources = [
     "main.dart",
     "foo.dart",
   ]
   deps = [
     "//mojo/public/dart",
     "//mojo/services/network/public/interfaces",
   ]
@@ skipping 32 matching lines @@
 
 The script `//mojo/public/tools/git/dart_pub_get.py` should be run before build
 time, e.g. as a "runhooks" action during `gclient sync`. The script traverses
 a directory tree looking for `pubspec.yaml` files. On finding one, in the
 containing directory, it runs `pub get`. This creates a "packages/" directory
 in the source tree adjacent to the `pubspec.yaml` file containing the downloaded
 Dart packages. `pub get` also creates a `pubspec.lock` file that locks down
 pub packages to specific versions. This `pubspec.lock` file must be checked in
 in order to have hermetic builds.
 
-During the build, The `dart_packaged_application` rule looks for a "packages/"
-directory, and copies its contents into the zip file.
+During the build, The `dartzip_packaged_application` rule looks for a
+"packages/" directory, and copies its contents into the zip file.
 
 ### Generated bindings
 
 The script `//mojo/public/tools/bindings/generators/mojom_dart_generator.py`
 and the templates under `//mojo/public/tools/bindings/generators/dart_templates`
 govern how `.mojom` files are compiled into Dart code.
 
 Consider the `network_error.mojom` file from the network services used by our
 "foo" example:
 
@@ skipping 29 matching lines @@
 
 Mojo's Dart content handler sets the package root for a Dart application to be
 the root directory of the unpacked zip file. Therefore, Dart sources in this
 application can use the following imports:
 
 ```dart
 import 'package:crypto/crypto.dart';
 import 'package:mojo/public/dart/application.dart';
 import 'package:mojom/mojo/network_error.mojom.dart';
 ```