feat(turbopack): add missing webpack context property (#7617)
### Description
https://vercel.slack.com/archives/C03EWR7LGEN/p1709598188040829?thread_ts=1709595370.496449&cid=C03EWR7LGEN
One of the loader uses context's `_module` property, puts an empty one
to avoid runtime exception at least.
Closes PACK-2656
