Avoid serializing if lockfile does not change

[ibraheemdev]

Summary

Avoid serializing and writing the lockfile if a cheap comparison shows that the contents have not changed.

Test Plan

Shaves ~10ms off of #4860 for me.

➜  transformers hyperfine "../../uv/target/profiling/uv lock" "../../uv/target/profiling/baseline lock" --warmup 3
Benchmark 1: ../../uv/target/profiling/uv lock
  Time (mean ± σ):     130.5 ms ±   2.5 ms    [User: 130.3 ms, System: 85.0 ms]
  Range (min … max):   126.8 ms … 136.9 ms    23 runs
 
Benchmark 2: ../../uv/target/profiling/baseline lock
  Time (mean ± σ):     140.5 ms ±   5.0 ms    [User: 142.8 ms, System: 85.5 ms]
  Range (min … max):   133.2 ms … 153.3 ms    21 runs
 
Summary
  ../../uv/target/profiling/uv lock ran
    1.08 ± 0.04 times faster than ../../uv/target/profiling/baseline lock