dotnet sln add not consistently discovering Project Type Guids for unknown project types

[baronfel]

Describe the bug

When adding projects to an slnx that use project extensions that are unknown to the vs-solutionpersistence library, we are not correctly discovering the DefaultProjectTypeGuid MSBuild property and applying that to the SolutionProjectModel's Type correctly.

To Reproduce

• clone https://github.com/ikvmnet/ikvm-maven
• rename src/IKVM.Maven.Sdk.Tasks.Java/IKVM.Maven.Sdk.Tasks.Java.csproj to src/IKVM.Maven.Sdk.Tasks.Java/IKVM.Maven.Sdk.Tasks.Java.msbuildproj
• convert the solution to slnx with dotnet sln migrate
• inspect the generated slnx and note there is a Type for the IKVM.Maven.Sdk.Tasks.Java project
• remove the project with dotnet sln remove src/IKVM.Maven.Sdk.Tasks.Java/IKVM.Maven.Sdk.Tasks.Java.msbuildproj
• add the project with dotnet sln add src/IKVM.Maven.Sdk.Tasks.Java/IKVM.Maven.Sdk.Tasks.Java.msbuildproj
• inspect the generated slnx and note there is no Type for the IKVM.Maven.Sdk.Tasks.Java project

Exceptions (if any)

during dotnet sln add, the following message is thrown:

Project 'D:\ikvm-maven\src\IKVM.Maven.Sdk.Tasks.Java\IKVM.Maven.Sdk.Tasks.Java.msbuildproj' has an unknown project type and cannot be added to the solution file. Contact your SDK provider for support.

Further technical details

• Include the output of dotnet --info: 9.0.2xx