Improved the runtime of DiffVulnerabilityResults

[neilnaveen]

• Improved the runtime of DiffVulnerabilityResults
• Precomputed the existence of Sources, Packages, and Vulnerabilities, which brings the time complexity down from O(n^2) to O(n).

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.05882% with 1 line in your changes missing coverage. Please review.

Project coverage is 66.01%. Comparing base (e35bd05) to head (43a04e4).
Report is 6 commits behind head on main.

Files	Patch %	Lines
internal/ci/vulnerability_result_diff.go	97.05%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1091      +/-   ##
==========================================
+ Coverage   65.94%   66.01%   +0.06%     
==========================================
  Files         159      159              
  Lines       12758    12784      +26     
==========================================
+ Hits         8413     8439      +26     
  Misses       3885     3885              
  Partials      460      460              

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[another-rex]

LGTM, thanks!