Fix Watchman on windows

[patroza]

Watchman returns / separated paths, but jest-haste-map uses path.sep which on windows equals \.

Summary

Addresses the issue finding tests on Windows with the latest watchman and Jest.
As a follow-up to #3887

[codecov-io]

Codecov Report

Merging #4018 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #4018   +/-   ##
=======================================
  Coverage   60.28%   60.28%           
=======================================
  Files         196      196           
  Lines        6764     6764           
  Branches        6        6           
=======================================
  Hits         4078     4078           
  Misses       2683     2683           
  Partials        3        3

Impacted Files	Coverage Δ
packages/jest-haste-map/src/crawlers/watchman.js	90.74% <100%> (ø)	⬆️

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[github-actions]

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