C#: Allow project source files to be separated into a sub project folder.

[exts]

Godot version:
3.2

OS/device including version:
All

Issue description:
Currently all C# projects are forced to be in the root project folder meaning the default namespace will always be that folder and most code is put into sub folders. When that happens your src, code, project, scripts, etc... subfolder where you store your scripts and other source code now is forced to be added into the namespace path eg.: ProjectName.Scripts which ideally despite scripts not needing namespaces, you would want your scripts on the root namespace which is just ProjectName.

I was messing around with jetbrains console project and realized there's a feature that allows you to set this in the sln file. For instance:

Microsoft Visual Studio Solution File, Format Version 12.00
Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "ConsoleApplication6", "ConsoleApplication6\ConsoleApplication6.csproj", "{D7E527AF-F5E5-45A0-AAE6-B4C46ED0347D}"
EndProject
Global
    GlobalSection(SolutionConfigurationPlatforms) = preSolution
        Debug|Any CPU = Debug|Any CPU
        Release|Any CPU = Release|Any CPU
    EndGlobalSection
    GlobalSection(ProjectConfigurationPlatforms) = postSolution
        {D7E527AF-F5E5-45A0-AAE6-B4C46ED0347D}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
        {D7E527AF-F5E5-45A0-AAE6-B4C46ED0347D}.Debug|Any CPU.Build.0 = Debug|Any CPU
        {D7E527AF-F5E5-45A0-AAE6-B4C46ED0347D}.Release|Any CPU.ActiveCfg = Release|Any CPU
        {D7E527AF-F5E5-45A0-AAE6-B4C46ED0347D}.Release|Any CPU.Build.0 = Release|Any CPU
    EndGlobalSection
EndGlobal

Now you could try manually editing this to your project sln, but building in editor complains if you move the .mono, .csproj to the sub folder.

Suggestion:
Allow c# projects to have a sub folder inside the project folder with the same name be the root of our project. This would make separating the c# much cleaner and organize the code base better.

Just a simple checkbox asking if the project source be in a subfolder ProjectName\ProjectName and when enabled the editor will look in that folder for the source as the root rather than the root project folder.

[mhilbrunner]

Feature and improvement proposals for the Godot Engine are now being discussed and reviewed in a dedicated Godot Improvement Proposals (GIP) (godotengine/godot-proposals) issue tracker. The GIP tracker has a detailed issue template designed so that proposals include all the relevant information to start a productive discussion and help the community assess the validity of the proposal for the engine.

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