Implement PackAsAnalyzer convention to include source generators in NuGet packages

[qodo-free-for-open-source-projects]

Description

• Source generator DLLs were missing from NuGet packages because PackAsAnalyzer="true" is a custom convention from dotnet/runtime that is not part of the .NET SDK and was silently ignored
• Consumers of NuGet packages had no source generation capability because the analyzer DLLs were not included in the analyzers/dotnet/cs/ folder
• Need to implement the convention properly in the build system to ensure generators are packaged correctly

Deliverables

• Build-time resolution: Create MSBuild target that sets OutputItemType=Analyzer on ProjectReferences marked with PackAsAnalyzer="true" before compilation
• Pack-time inclusion: Create MSBuild target that resolves generator DLLs via GetTargetPath and includes them in analyzers/dotnet/cs/ folder in NuGet packages
• Global application: Import the new targets in Directory.Build.props so all projects using PackAsAnalyzer automatically benefit from the convention

[linear]

EVE-225