Dart: Reorganize files

This change attempts to provide a more rational structure for
Dart-related code. As much as possible code is moved under //mojo/dart.
The layout is now:

apptests/
  dart_apptests/ (moved from //services/dart/dart_apptests)
  test_apps/  (moved from //services/dart/tests)
embedder/
  snapshotter/ (moved from //mojo/dart/dart_snapshotter)
packages/  (no changes)
tools/  (no changes)
unittests/
  embedder_tester/  (moved from //mojo/dart/embedder/test)
  embedder_tests/  (moved from //mojo/dart/test)
  http_load_test/  (moved from //mojo/dart/http_load_test)
  observatory_test/  (moved from //mojo/dart/observatory_test)
  observatory_tester/  (moved from //mojo/dart/observatory_tester)

I've also consolidated .gitignore rules under //.gitignore and deleted
stray pubspec.lock files.

R=johnmccutchan@google.com

Review URL: https://codereview.chromium.org/1545483003 .
diff --git a/build/module_args/dart.gni b/build/module_args/dart.gni
index e784261..d3c3eec 100644
--- a/build/module_args/dart.gni
+++ b/build/module_args/dart.gni
@@ -5,4 +5,5 @@
 # This variable should point to the Dart SDK.
 dart_sdk_root = "//third_party/dart-sdk/dart-sdk"
 
-dart_snapshotter_bin = "//mojo/dart/dart_snapshotter($host_toolchain)"
+dart_snapshotter_bin =
+    "//mojo/dart/embedder/snapshotter:dart_snapshotter($host_toolchain)"