Refactor import boundaries and API signatures for consistency across pyfuse modules

[Copilot]

This PR applies two cross-cutting code quality rules across the affected runtime modules: avoid inline imports except where needed to break cycles, and align argument naming/order for shared call paths.

• Import hygiene (inline imports removed where unnecessary)

  • Moved avoidable function-scoped imports to module scope in:
    • pyfuse/__main__.py
    • pyfuse/worker/worker.py
    • pyfuse/worker/remote.py
  • Kept lazy import only where it is required to avoid an import cycle (Graph inside submit_remote).

• Optional-backend import behavior preserved

  • Reworked backend modules to keep optional dependency ergonomics while reducing inline imports:
    • pyfuse/worker/backends/redis.py
    • pyfuse/worker/backends/rabbitmq.py
  • Added guarded module-level imports (try/except ImportError) and retained clear dependency error paths at backend initialization.

• Argument coherence across modules

  • Standardized remote submission override naming:
    • submit_remote(..., _backend=...) → submit_remote(..., backend=...)
  • Updated traced-call wiring to match (graph/tracing.py).
  • Aligned parameter ordering in serve(...) with worker construction conventions (import_to_package before auto_install).

# Before
await submit_remote(func, wrapper, *args, _backend=backend, **kwargs)

# After
await submit_remote(func, wrapper, *args, backend=backend, **kwargs)