Unable to specify a subdirectory for assembly loading

Start with the runtime spec documentation in Documentation/specs/corehost.md and compare its stated root-directory search behavior with app.runtimeconfig.json and app.runtimeconfig.dev.json. Reproduce the moved-DLL scenario and investigate whether additionalProbingPaths is intended to support private subdirectories. Done means establishing and testing a supported configuration path, or documenting that the behavior is unavailable.

Steps to reproduce

Build a project for .NET Core 3. Add some nuget package.
Create a "lib" subdirectory in the output folder.
Move some/all of the .dll files into "lib".
Program will fail to run, as it is unable to locate the moved assemblies.

Details

When building a program using the standard framework, which used app.exe.config for its runtime configuration, you could specify private subdirectories that would be searched when resolving assembly loading:

  <runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      <probing privatePath="lib" />
    </assemblyBinding>
  </runtime>

I have been unable to find any directives that would reproduce that behavior in the new app.runtimeconfig.json configuration. The runtime spec documentation only mentions searching the root application directory, and doesn't indicate any means of getting the runtime to search alternate directories (even with the presumed restriction of only being allowed to search private subdirectories).

Use of additionalProbingPaths in app.runtimeconfig.json and app.runtimeconfig.dev.json does not seem to work.

Environment data

dotnet --info output:

.NET Core SDK (reflecting any global.json):
Version: 3.0.100-preview6-012264
Commit: be3f0c1a03

Runtime Environment:
OS Name: Windows
OS Version: 6.1.7601
OS Platform: Windows
RID: win7-x64
Base Path: C:\Program Files\dotnet\sdk\3.0.100-preview6-012264\

Host (useful for support):
Version: 3.0.0-preview6-27804-01
Commit: fdf81c6faf

.NET Core SDKs installed:
1.1.14 [C:\Program Files\dotnet\sdk]
2.1.602 [C:\Program Files\dotnet\sdk]
2.1.604 [C:\Program Files\dotnet\sdk]
2.1.700 [C:\Program Files\dotnet\sdk]
2.1.800-preview-009696 [C:\Program Files\dotnet\sdk]
2.2.202 [C:\Program Files\dotnet\sdk]
2.2.204 [C:\Program Files\dotnet\sdk]
2.2.300 [C:\Program Files\dotnet\sdk]
2.2.400-preview-010219 [C:\Program Files\dotnet\sdk]
3.0.100-preview6-012264 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
Microsoft.AspNetCore.All 2.1.9 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.11 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.2.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.2.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.9 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.11 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.2.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.2.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.0.0-preview6.19307.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 1.0.16 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 1.1.13 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.11 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.2.3 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.2.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.0.0-preview6-27804-01 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 3.0.0-preview6-27804-01 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]