[cmd2] Move .cpp files to subproject source root (#8967)

This matches all of the other subprojects.

Signed-off-by: Zach Harel <zach@zharel.me>

commandsv2/src/main/native/cpp/SubsystemBase.cpp

@@ -0,0 +1,75 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#include "wpi/commands2/SubsystemBase.hpp"
+
+#include <string>
+
+#include "wpi/commands2/Command.hpp"
+#include "wpi/commands2/CommandScheduler.hpp"
+#include "wpi/util/Demangle.hpp"
+#include "wpi/util/sendable/SendableBuilder.hpp"
+#include "wpi/util/sendable/SendableRegistry.hpp"
+
+using namespace wpi::cmd;
+
+SubsystemBase::SubsystemBase() {
+  wpi::util::SendableRegistry::Add(this, GetTypeName(*this));
+  CommandScheduler::GetInstance().RegisterSubsystem({this});
+}
+
+SubsystemBase::SubsystemBase(std::string_view name) {
+  wpi::util::SendableRegistry::Add(this, name);
+  CommandScheduler::GetInstance().RegisterSubsystem({this});
+}
+
+void SubsystemBase::InitSendable(wpi::util::SendableBuilder& builder) {
+  builder.SetSmartDashboardType("Subsystem");
+  builder.AddBooleanProperty(
+      ".hasDefault", [this] { return GetDefaultCommand() != nullptr; },
+      nullptr);
+  builder.AddStringProperty(
+      ".default",
+      [this]() -> std::string {
+        auto command = GetDefaultCommand();
+        if (command == nullptr) {
+          return "none";
+        }
+        return command->GetName();
+      },
+      nullptr);
+  builder.AddBooleanProperty(
+      ".hasCommand", [this] { return GetCurrentCommand() != nullptr; },
+      nullptr);
+  builder.AddStringProperty(
+      ".command",
+      [this]() -> std::string {
+        auto command = GetCurrentCommand();
+        if (command == nullptr) {
+          return "none";
+        }
+        return command->GetName();
+      },
+      nullptr);
+}
+
+std::string SubsystemBase::GetName() const {
+  return wpi::util::SendableRegistry::GetName(this);
+}
+
+void SubsystemBase::SetName(std::string_view name) {
+  wpi::util::SendableRegistry::SetName(this, name);
+}
+
+std::string SubsystemBase::GetSubsystem() const {
+  return wpi::util::SendableRegistry::GetSubsystem(this);
+}
+
+void SubsystemBase::SetSubsystem(std::string_view name) {
+  wpi::util::SendableRegistry::SetSubsystem(this, name);
+}
+
+void SubsystemBase::AddChild(std::string name, wpi::util::Sendable* child) {
+  wpi::util::SendableRegistry::Add(child, GetSubsystem(), name);
+}