fix(core): join with os native sep in resolver

[pi0]

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

After investigation for the root cause of #7097, it seems node CJS cache entry ids are exactly how they are being required and something like require('c:\\foo\\bar/index.js) is also valid but HMR watcher get os native path (c:\\foo\\bar\\index.js). This issue was not discovered until yet because node.js for windows supports mixed path separators.

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly. (PR: #)
• I have added tests to cover my changes (if not applicable, please state why)
• All new and existing tests are passing.

[codecov-io]

Codecov Report

Merging #7131 into dev will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev    #7131      +/-   ##
==========================================
+ Coverage   62.75%   62.76%   +0.01%     
==========================================
  Files          84       84              
  Lines        3399     3400       +1     
  Branches      928      928              
==========================================
+ Hits         2133     2134       +1     
  Misses       1018     1018              
  Partials      248      248              

Flag	Coverage Δ
#unittests	62.76% <100.00%> (+0.01%)	⬆️

Impacted Files	Coverage Δ
packages/core/src/resolver.js	97.33% <100.00%> (+0.03%)	⬆️

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