Replace Reference/PackageDependencies feature checks with ReferencePath filename checks

[Copilot]

The .targets file was using two MSBuild item group checks per assembly (Reference + PackageDependencies by Identity) to detect whether a dependency is present. This replaces both with a single ReferencePath check by Filename metadata.

Closes #88

Changes

• PolyShim/PolyShim.targets: For every feature, removed the paired Reference/PackageDependencies identity checks and replaced with a single ReferencePath filename check:

<!-- Before (two conditions per assembly) -->
<FeatureArrayPool
  Condition="@(Reference-&gt;AnyHaveMetadataValue('Identity', 'System.Buffers'))"
  >true</FeatureArrayPool>
<FeatureArrayPool
  Condition="@(PackageDependencies-&gt;AnyHaveMetadataValue('Identity', 'System.Buffers'))"
  >true</FeatureArrayPool>

<!-- After (single condition) -->
<FeatureArrayPool
  Condition="@(ReferencePath-&gt;AnyHaveMetadataValue('Filename', 'System.Buffers'))"
  >true</FeatureArrayPool>

Features that previously checked two assemblies (e.g. FeatureHashCode checking both System.HashCode and Microsoft.Bcl.HashCode, FeatureHttpClient, FeatureTask) retain two conditions — one per assembly — but each now uses ReferencePath/Filename rather than the old pair pattern.

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[Tyrrrz]

@copilot closes #88

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.31%. Comparing base (468caa7) to head (90d89c1).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #87      +/-   ##
==========================================
- Coverage   85.37%   85.31%   -0.06%     
==========================================
  Files         220      220              
  Lines        3603     3603              
  Branches      314      314              
==========================================
- Hits         3076     3074       -2     
- Misses        455      457       +2     
  Partials       72       72              

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[Copilot]

Pull request overview

Updates PolyShim’s MSBuild feature-detection logic to use resolved references rather than separate Reference + PackageDependencies identity checks, simplifying how feature flags are determined during the build.

Changes:

• Replaced paired @(Reference) / @(PackageDependencies) Identity checks with a single @(ReferencePath) Filename check per dependency.
• Kept multi-assembly feature detection (e.g., HashCode/HttpClient/Task) as multiple conditions, but migrated each to the new ReferencePath approach.

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