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/Models/Object.cs

@@ -0,0 +1,18 @@
+namespace SourceGit.Models
+{
+    public enum ObjectType
+    {
+        None,
+        Blob,
+        Tree,
+        Tag,
+        Commit,
+    }
+
+    public class Object
+    {
+        public string SHA { get; set; }
+        public ObjectType Type { get; set; }
+        public string Path { get; set; }
+    }
+}