Workaround shutil.rmtree limitation on Windows

By default, shutil.rmtree raises an exception when deleting readonly
files on Windows.

Replace all shutil.rmtree with platform_utils.rmtree, which adds an
error handler to make files read-write when they can't be deleted.

Change-Id: I9cfea9a7b3703fb16a82cf69331540c2c179ed53
diff --git a/subcmds/init.py b/subcmds/init.py
index 65dfd1f..e647091 100644
--- a/subcmds/init.py
+++ b/subcmds/init.py
@@ -17,7 +17,6 @@
 import os
 import platform
 import re
-import shutil
 import sys
 
 from pyversion import is_python3
@@ -35,6 +34,7 @@
 from project import SyncBuffer
 from git_config import GitConfig
 from git_command import git_require, MIN_GIT_VERSION
+import platform_utils
 
 class Init(InteractiveCommand, MirrorSafeCommand):
   common = True
@@ -252,7 +252,7 @@
       # Better delete the manifest git dir if we created it; otherwise next
       # time (when user fixes problems) we won't go through the "is_new" logic.
       if is_new:
-        shutil.rmtree(m.gitdir)
+        platform_utils.rmtree(m.gitdir)
       sys.exit(1)
 
     if opt.manifest_branch: