PVF: Refactor workers into separate crates, remove host dependency (#7253)

* PVF: Refactor workers into separate crates, remove host dependency

* Fix compile error

* Remove some leftover code

* Fix compile errors

* Update Cargo.lock

* Remove worker main.rs files

I accidentally copied these from the other PR. This PR isn't intended to
introduce standalone workers yet.

* Address review comments

* cargo fmt

* Update a couple of comments

* Update log targets