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/Tag.cs

@@ -0,0 +1,49 @@
+using System.Collections.Generic;
+using System.IO;
+
+namespace SourceGit.Commands
+{
+    public static class Tag
+    {
+        public static bool Add(string repo, string name, string basedOn, string message)
+        {
+            var cmd = new Command();
+            cmd.WorkingDirectory = repo;
+            cmd.Context = repo;
+            cmd.Args = $"tag -a {name} {basedOn} ";
+
+            if (!string.IsNullOrEmpty(message))
+            {
+                string tmp = Path.GetTempFileName();
+                File.WriteAllText(tmp, message);
+                cmd.Args += $"-F \"{tmp}\"";
+            }
+            else
+            {
+                cmd.Args += $"-m {name}";
+            }
+
+            return cmd.Exec();
+        }
+
+        public static bool Delete(string repo, string name, List<Models.Remote> remotes)
+        {
+            var cmd = new Command();
+            cmd.WorkingDirectory = repo;
+            cmd.Context = repo;
+            cmd.Args = $"tag --delete {name}";
+            if (!cmd.Exec())
+                return false;
+
+            if (remotes != null)
+            {
+                foreach (var r in remotes)
+                {
+                    new Push(repo, r.Name, name, true).Exec();
+                }
+            }
+
+            return true;
+        }
+    }
+}