Use Path.Join instead of Path.Combine

[JeremyKuhne]

It is faster, and more predictable. Most changes are in test code.

Tweak behavior in AssemblyNamesTypeResolutionService to use API based program files and add separator for proper contains checking. No need to use both paths, it will always be current process specific.

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

Attention: Patch coverage is 94.18605% with 5 lines in your changes missing coverage. Please review.

Project coverage is 61.02481%. Comparing base (159a161) to head (e241b89).
Report is 2 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #13090         +/-   ##
===================================================
- Coverage   61.03240%   61.02481%   -0.00760%     
===================================================
  Files           1539        1539                 
  Lines         158039      158039                 
  Branches       14721       14721                 
===================================================
- Hits           96455       96443         -12     
- Misses         60889       60901         +12     
  Partials         695         695                 

Flag	Coverage Δ
Debug	61.02481% <94.18605%> (-0.00760%)	⬇️
integration	10.72935% <0.00000%> (ø)
production	38.81372% <100.00000%> (-0.01246%)	⬇️
test	95.67041% <94.11765%> (ø)
unit	36.23846% <100.00000%> (-0.01246%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

🚀 New features to boost your workflow:

• ❄ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.