Use fast path in modulefinder more often by hauntsaninja · Pull Request #17950 · python/mypy

@hauntsaninja

See #17948

This is about 1.06x faster on `mypy -c 'import torch'` (in both the
clean and openai environments)
- 19.094 -> 17.896 
- 34.161 -> 32.214

```
λ hyperfine -w 1 -M 3 '/tmp/mypy_primer/timer_mypy_36738b392/venv/bin/mypy  -c "import torch" --no-incremental --python-executable clean/bin/python'
Benchmark 1: /tmp/mypy_primer/timer_mypy_36738b392/venv/bin/mypy  -c "import torch" --no-incremental --python-executable clean/bin/python
  Time (mean ± σ):     17.896 s ±  0.130 s    [User: 16.472 s, System: 1.408 s]
  Range (min … max):   17.757 s … 18.014 s    3 runs

 λ hyperfine -w 1 -M 3 '/tmp/mypy_primer/timer_mypy_36738b392/venv/bin/mypy  -c "import torch" --no-incremental --python-executable /opt/oai/bin/python' 
Benchmark 1: /tmp/mypy_primer/timer_mypy_36738b392/venv/bin/mypy  -c "import torch" --no-incremental --python-executable /opt/oai/bin/python
  Time (mean ± σ):     32.214 s ±  0.106 s    [User: 29.468 s, System: 2.722 s]
  Range (min … max):   32.098 s … 32.305 s    3 runs
```