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/Submodule.cs

@@ -0,0 +1,55 @@
+using System;
+
+namespace SourceGit.Commands
+{
+    public class Submodule : Command
+    {
+        public Submodule(string repo)
+        {
+            WorkingDirectory = repo;
+            Context = repo;
+        }
+
+        public bool Add(string url, string relativePath, bool recursive, Action<string> outputHandler)
+        {
+            _outputHandler = outputHandler;
+            Args = $"submodule add {url} {relativePath}";
+            if (!Exec())
+                return false;
+
+            if (recursive)
+            {
+                Args = $"submodule update --init --recursive -- {relativePath}";
+                return Exec();
+            }
+            else
+            {
+                Args = $"submodule update --init -- {relativePath}";
+                return true;
+            }
+        }
+
+        public bool Update()
+        {
+            Args = $"submodule update --rebase --remote";
+            return Exec();
+        }
+
+        public bool Delete(string relativePath)
+        {
+            Args = $"submodule deinit -f {relativePath}";
+            if (!Exec())
+                return false;
+
+            Args = $"rm -rf {relativePath}";
+            return Exec();
+        }
+
+        protected override void OnReadline(string line)
+        {
+            _outputHandler?.Invoke(line);
+        }
+
+        private Action<string> _outputHandler;
+    }
+}