fix: lazily read package.json.exports for shared resolvers

[Boshen]

relates #135

This PR removes all encapsulations around imports / exports map so we can dynamically retrieve and evaluate their values :-(

[codecov]

Codecov Report

Attention: Patch coverage is 98.46154% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 95.08%. Comparing base (b2e2094) to head (be4b89d).

Files	Patch %	Lines
src/lib.rs	97.87%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #137      +/-   ##
==========================================
+ Coverage   95.01%   95.08%   +0.07%     
==========================================
  Files          11       11              
  Lines        2246     2238       -8     
==========================================
- Hits         2134     2128       -6     
+ Misses        112      110       -2     

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[codspeed-hq]

CodSpeed Performance Report

Merging #137 will not alter performance

_{Comparing exports (be4b89d) with main (b2e2094)}

Summary

✅ 2 untouched benchmarks