[Bug #18599]

`vm->loaded_features` and `vm->loaded_features_snapshot` both share the
same root. When a feature is pushed into `loaded_features`, the sharing
is broken and `loaded_features` is copied.

So an application requiring 1000 files, will allocate 1000 arrays of
increasing size, which is very wasteful.

To avoid this, we first clear the snapshot, so that `loaded_features`
can directly be pushed into.

Co-Authored-By: Peter Zhu <peter.zhu@shopify.com>