resolver: delete unused ModKeyError, FileSystemError, ReverseResolution, and dead helper fns (#31548)

Dead-code sweep of `bun_resolver`. None referenced outside their own
definitions in the Rust tree.

`cargo check --workspace` passes.

## Why this code exists

| Item | Zig original | Zig callers | Status |
|---|---|---|---|
| `ModKeyError` | `src/resolver/fs.zig:841` | 0 — `ModKey.generate`
(`:887`) returns `anyerror!`, not `ModKeyError!` | **dead in Zig too** |
| `ModKey::SAFETY_GAP` | `fs.zig:912` (`SafetyGap`) | 0 — defined but
never read, even inside `ModKey.generate` | **dead in Zig too** |
| `FileSystemError` | none (0 hits in `*.zig`) | n/a | **Rust-port
artifact** — defined only at `fs.rs` with a `TODO(port): impl From<...>`
note |
| `kind_from_absolute` | `fs.zig:1308` (`kindFromAbsolute`) | 0 | **dead
in Zig too** |
| `ReverseResolution` | `src/resolver/package_json.zig:1496` | 0 —
return type of
`resolveExportsReverse`/`resolveImportsExportsReverse`/`resolveTargetReverse`
(`:1993,2007,2041`), all of which have 0 external callers | **dead in
Zig too** |
| `hash_module` | `package_json.zig:1081` (`hashModule`) | 0 | **dead in
Zig too** |
| `has_parent_package` | `src/resolver/dir_info.zig:56`
(`hasParentPackage`) | 0 | **dead in Zig too** |

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Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>