Retire legacy FileSystem trait

[Jay Bosamiya (Microsoft) (jaybosamiya-ms)]

This PR mechanically removes the legacy FileSystem trait, so that everything is now based on the new Backend-based interface (#887). The relevant north interface now sits as inherent methods of Resolver.

[Jay Bosamiya (Microsoft) (jaybosamiya-ms)]

More tweaks to the filesystem in future PRs, this one is mechanical but touches a lot of files so I've spun it out as its own PR to simplify reviewing

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

Click for details

--- failure trait_missing: pub trait removed or renamed ---

Description:
A publicly-visible trait cannot be imported by its prior path. A `pub use` may have been removed, or the trait 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/v0.50.0/src/lints/trait_missing.ron

Failed in:
  trait litebox::fs::FileSystem, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/6e21ee1f8110d60a251b3057c5092d1ec6161a18/litebox/src/fs/mod.rs:51

--- failure method_requires_different_generic_type_params: method now requires a different number of generic type parameters ---

Description:
A method now requires a different number of generic type parameters than it used to. Uses of this method that supplied the previous number of generic types will be broken.
        ref: https://doc.rust-lang.org/reference/items/generics.html
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.50.0/src/lints/method_requires_different_generic_type_params.ron

Failed in:
  litebox_shim_linux::LinuxShimBuilder::build takes 0 generic types instead of 1, in /home/runner/work/litebox/litebox/litebox_shim_linux/src/lib.rs:219

--- failure trait_missing: pub trait removed or renamed ---

Description:
A publicly-visible trait cannot be imported by its prior path. A `pub use` may have been removed, or the trait 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/v0.50.0/src/lints/trait_missing.ron

Failed in:
  trait litebox_shim_linux::ShimFS, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/6e21ee1f8110d60a251b3057c5092d1ec6161a18/litebox_shim_linux/src/lib.rs:65

--- failure type_allows_fewer_generic_type_params: type now allows fewer generic type parameters ---

Description:
A type now allows fewer generic type parameters than it used to. Uses of this type that supplied all previously-supported generic types will be broken.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#trait-new-parameter-no-default
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.50.0/src/lints/type_allows_fewer_generic_type_params.ron

Failed in:
  Struct LinuxShimEntrypoints allows 2 -> 1 generic types in /home/runner/work/litebox/litebox/litebox_shim_linux/src/lib.rs:123
  Struct LoadedProgram allows 2 -> 1 generic types in /home/runner/work/litebox/litebox/litebox_shim_linux/src/lib.rs:345
  Struct LinuxShim allows 2 -> 1 generic types in /home/runner/work/litebox/litebox/litebox_shim_linux/src/lib.rs:238