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/GC.cs

@@ -0,0 +1,23 @@
+using System;
+
+namespace SourceGit.Commands
+{
+    public class GC : Command
+    {
+        public GC(string repo, Action<string> outputHandler)
+        {
+            _outputHandler = outputHandler;
+            WorkingDirectory = repo;
+            Context = repo;
+            TraitErrorAsOutput = true;
+            Args = "gc";
+        }
+
+        protected override void OnReadline(string line)
+        {
+            _outputHandler?.Invoke(line);
+        }
+
+        private readonly Action<string> _outputHandler;
+    }
+}