Allow Flutter to be vendored as a submodule (#14157)

When Flutter is vendored as a submodule, `.git` is a File whose contents
are a pointer to the Git directory. This change allows you to pin your
app to a certain version of Flutter via submodules.

bin/flutter

@@ -126,7 +126,7 @@ if ! hash git 2>/dev/null; then
   exit 1
 fi
 # Test if the flutter directory is a git clone (otherwise git rev-parse HEAD would fail)
-if [ ! -d "$FLUTTER_ROOT/.git" ]; then
+if [ ! -e "$FLUTTER_ROOT/.git" ]; then
   echo "Error: The Flutter directory is not a clone of the GitHub project."
   exit 1
 fi