Refactoring the method that fetches the project context in the Solution PM UI

[jebriede]

Refactoring the method that fetches the project context in the Solution PM UI to return only a single package and simplify the caller logic

Bug

Fixes: https://github.com/NuGet/Home/issues/13216

Description

Addressing feedback from the original pull request for the Transitive Packages in Solution PM UI feature: https://github.com/NuGet/NuGet.Client/pull/6044#discussion_r1774288377. This is a better design to only return one package context object and simplifies the caller logic in determining if the package is top-level or transitive. Removed an outdated comment on the method, and renamed the method to more accurately reflect its purpose.

PR Checklist

• [x] Meaningful title, helpful description and a linked NuGet/Home issue
• [x] ~Added tests~ - Refactoring existing method. No new functionality to test.
• [x] Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc.