Add support for specifying folders that aren't namespace providers.

[tacosontitan]

IDE0130 helps keep namespaces equivalent to the folder structure, and I completely agree with it. However, on occasion, I have a folder that exists purely for organizational benefit and should not be part of the namespace. A good example is the framework project I maintain at work where we define many extensions for IServiceCollection as part of our Runtime.DependencyInjection namespace to support our ecosystem, and these are broken down into separate files:

Each of these files is defined as:

namespace Runtime.DependencyInjection;

public static partial class ServiceCollectionExtensions
{
    ...
}

Unfortunately for this use case, according to IDE0130, the namespace should be adjusted to add the SeviceCollectionExtensions folder effectively becoming Runtime.DependencyInjection.ServiceCollectionExtensions; this forces us to import two namespaces when working with our dependency injection extensions simply because of an organizational decision we made.

JetBrains products have a way for us to specify that a folder isn't a namespace provider, and I'd love to be capable of specifying that a folder is not a namespace provider at the .csproj level, so something like:

<ItemGroup Label="Pardoned Namespace Violators">
    <NamespaceProvider "DependencyInjection/ServiceCollectionExtensions">false</NamespaceProvider>
</ItemGroup>

I initially thought maybe Folder or None could be used with an additional property like:

<ItemGroup Label="Pardoned Namespace Violators">
    <Folder Include="DependencyInjection/ServiceCollectionExtensions" NamespaceProvider="False"/>
</ItemGroup>

However, given that I don't have enough experience with those yet, please take that with a grain of salt; this discussion is mostly to bring the idea forward for community feedback.

[davidwengier]

Related (or dupe of?): dotnet/project-system#5372
Also: #45494