Migrate 9p file system to new backend

[jaybosamiya-ms]

This PR switches our 9p filesystem to the new core file system design (see #887). Like prior migrations, it adds a new backend, migrates all old usages to a resolver-backed one to use the new backend, and then removes the old FileSystem object.

Additionally, as a drive-by-fix, the old read_dir's check reports every entry as a regular file (because it looked at e.typ rather than e.qid.typ); the new backend fixes this.

[github-actions]

ℹ️ Note: This semver check was run against the jayb/reduced-locking-on-iter branch, not main or ulitebox.

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

Click for details

--- failure struct_missing: pub struct removed or renamed ---

Description:
A publicly-visible struct cannot be imported by its prior path. A `pub use` may have been removed, or the struct itself may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.49.0/src/lints/struct_missing.ron

Failed in:
  struct litebox::fs::nine_p::FileSystem, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-jayb_reduced_locking_on_iter/3fdf72ae91f6700a600cc7a8cc3921b5686fefec/litebox/src/fs/nine_p/mod.rs:271

[jaybosamiya-ms]

Similar to other migrations, it is easiest to review one commit at a time.