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/Commands/Init.cs

@@ -0,0 +1,12 @@
+namespace SourceGit.Commands
+{
+    public class Init : Command
+    {
+        public Init(string ctx, string dir)
+        {
+            Context = ctx;
+            WorkingDirectory = dir;
+            Args = "init -q";
+        }
+    }
+}