Update ProjectSystemCache to be case insensitive for _primaryCache

[jgonz120]

Bug

Fixes: https://github.com/NuGet/Client.Engineering/issues/2829

Description

Projects information was being added/updated with different cases, causes our apex tests to fail. The primary key for the _primaryCache is projectNames.FullName which is the full path and filename of the project. Switching the dictionary to ignore case allows the project to be added to the single entry instead of creating a new one.

PR Checklist

• [x] Meaningful title, helpful description and a linked NuGet/Home issue
• [x] Added tests
• [x] ~~Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc.~~