Implement valuedefinedincontext for intercepted properties

[adamint]

This completes the project-system side of #7825 by overriding InterceptedProjectProperties to check if IInterceptingPropertyValueProvider2's GetProjectFileMSBuildPropertyNamesAsync returns a value that is directly defined in the project file.

[adamint]

@tmeschter based on what you said, I renamed to IsValueDefinedInContext so this can eventually support myapp/launch profiles