[WIP] Refactor PackageSpecFactory to use constants for property and item names

[Copilot]

This PR refactors the PackageSpecFactory class to use constants from ProjectBuildProperties, ProjectItems, and ProjectItemProperties instead of string literals for MSBuild property and item names. This improves maintainability and reduces duplication, following NuGet coding guidelines.

• Replaced string literals such as "TargetFramework", "PackageReference", "IncludeAssets", etc. with the appropriate constants.
• Added necessary using directive for NuGet.ProjectManagement.
• No functional changes, only code quality improvements.

No errors were introduced by this change.

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[zivkan]

@copilot I still see about 30 more instances of .GetProperty(". Please have another look. If the properties don't have constances, please add a constant to ProjectBuildProperties or ProjectItemProperties as needed.

[Copilot]

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[zivkan]

@copilot I can still see about 30 more calls to GetProperty that pass a string literal. Please have another look, and create new constants in ProjectBuildProperties and ProjectItemProperties as needed.

[Copilot]

@zivkan Unfortunately I hit an unexpected error while processing your comment. I've automatically reported this to GitHub.

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If you want to contact GitHub about this error, please mention the following identifier so they can better serve you: ce536a11-467f-499c-89cb-c601882f9295

Sorry for the inconvenience!