Move impeller-cmake-example to flutter/third_party. (#53589)

... as part of buildmoot.

I _think_ this is all that is needed but am not confident.

Author: matanlurey

DEPS

@@ -1008,7 +1008,7 @@ deps = {
      'dep_type': 'cipd',
    },
 
-  'src/third_party/impeller-cmake-example': {
+  'src/flutter/third_party/impeller-cmake-example': {
      'url': Var('flutter_git') + '/third_party/impeller-cmake-example.git' + '@' + '9f8298ec31dcbebbf019bd487888166abf2f55e6',
      'condition': 'download_impeller_cmake_example',
   },
@@ -1136,7 +1136,7 @@ hooks = [
       'python3',
       'src/flutter/ci/impeller_cmake_build_test.py',
       '--path',
-      'third_party/impeller-cmake-example',
+      'flutter/third_party/impeller-cmake-example',
       '--setup',
     ]
   },

tools/gn

@@ -824,7 +824,7 @@ def to_gn_wasm_args(args, gn_args):
 
 
 def run_impeller_cmake(args):
-  impeller_cmake_dir = os.path.join('third_party', 'impeller-cmake-example')
+  impeller_cmake_dir = os.path.join('flutter', 'third_party', 'impeller-cmake-example')
   if not os.path.isdir(os.path.join(SRC_ROOT, impeller_cmake_dir)):
     print('The Impeller cmake example directory "{}" does not exist'.format(impeller_cmake_dir))
     return 1