[build] [bug] Fix a bug of skbuild that loses the root package_dir (#4875)

Author: frostming

setup.py

@@ -64,6 +64,12 @@ def remove_tmp(taichi_dir):
 
 
 class EggInfo(egg_info):
+    def finalize_options(self, *args, **kwargs):
+        if '' not in self.distribution.package_dir:
+            # XXX: skbuild loses the root package dir
+            self.distribution.package_dir[''] = package_dir
+        return super().finalize_options(*args, **kwargs)
+
     def run(self):
         taichi_dir = os.path.join(package_dir, 'taichi')
         remove_tmp(taichi_dir)