project: reorganize the structure of the project.

* remove dotnet-tool.json because the project does not rely on any dotnet tools.
* remove Directory.Build.props because the solution has only one project.
* move src/SourceGit to src. It's not needed to put all sources into a subfolder of src since there's only one project.

src/Views/CreateGroup.axaml.cs

@@ -0,0 +1,12 @@
+using Avalonia.Controls;
+
+namespace SourceGit.Views
+{
+    public partial class CreateGroup : UserControl
+    {
+        public CreateGroup()
+        {
+            InitializeComponent();
+        }
+    }
+}