Related issues:

dotnet/winforms#638
dotnet/project-system#4807
https://github.com/dotnet/corefx/issues/34190

In SDK projects and new project system, we have eliminated the need for DependentUpon to make the appropriate file nesting in the IDE tree. However, there is a place where the build actually uses DependentUpon to:

1. Locate a source file, and parse (!) it to get first class name and namespace
2. Generate .resources accordingly.

Let first me say that this whole design is terrible and it is absolutely insane that we have code to find the class and namespace name in msbuild:

https://github.com/microsoft/msbuild/blob/f309171ed821eb66cb635403e1fbff41ea22ae9c/src/Tasks/CSharpParserUtilities.cs#L15-L20

But people and features in VS rely on this and it has been a consistent source of feedback in moving to .NET Core 3.0. So I give up and propose:

1. Add a boolean property that, when true, causes us to look for the source file by convention when DependentUpon metadata is not found.

2. Set that property by default for .NET Core App 3.0 TFM.

cc @rainersigwald @davkean @livarcocc