refactor: extract realpath native into a separate module

[SimenB]

Summary
See #5085 (comment)

Quick and dirty extraction

Test plan
Green CI

[codecov-io]

Codecov Report

Merging #5097 into master will decrease coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #5097      +/-   ##
=========================================
- Coverage   60.66%   60.6%   -0.07%     
=========================================
  Files         201     201              
  Lines        6694    6688       -6     
  Branches        4       4              
=========================================
- Hits         4061    4053       -8     
- Misses       2633    2635       +2

Impacted Files	Coverage Δ
packages/jest-util/src/index.js	100% <ø> (+9.52%)	⬆️
packages/jest-runtime/src/script_transformer.js	84.52% <100%> (-2.31%)	⬇️

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[cpojer]

Thank you!

[github-actions]

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