Optimize flattening in apache airflow workspace (#11313)
## Motivation
No-op `uv lock` in apache airflow
(891c67f210ab7c877d1f00ea6ea3d3cdbb0e96ef) is slow, which makes `uv run`
slow, too.
Reference project:
```
$ hyperfine "uv run python -c \"print('hi')\""
Benchmark 1: uv run python -c "print('hi')"
Time (mean ± σ): 16.3 ms ± 1.5 ms [User: 9.8 ms, System: 6.4 ms]
Range (min … max): 13.0 ms … 20.0 ms 186 runs
```
Apache airflow before:
```
$ hyperfine "uv run python -c \"print('hi')\""
Benchmark 1: uv run python -c "print('hi')"
Time (mean ± σ): 161.0 ms ± 5.2 ms [User: 135.3 ms, System: 24.1 ms]
Range (min … max): 155.0 ms … 176.3 ms 18 runs
```
## Optimization
`FlatRequiresDist::from_requirements` is taking 50% of main thread
runtime.
Before:
After both commits:
Apache airflow after the first commit:
```
$ hyperfine "uv-profiling run python -c \"print('hi')\""
Benchmark 1: uv-profiling run python -c "print('hi')"
Time (mean ± σ): 122.3 ms ± 5.4 ms [User: 96.1 ms, System: 24.7 ms]
Range (min … max): 114.0 ms … 133.2 ms 23 runs
```
Apache airflow after the second commit:
```
$ hyperfine "uv-profiling run python -c \"print('hi')\""
Benchmark 1: uv-profiling run python -c "print('hi')"
Time (mean ± σ): 108.5 ms ± 3.4 ms [User: 83.2 ms, System: 24.2 ms]
Range (min … max): 103.6 ms … 119.9 ms 28 runs
```
