refactor(build): restructure package layout and fix CMake build directory restoration

- Update pybind_src to point to project root instead of dm_imu subdirectory
- Change CMakeExtension module from "imu_py" to "dm_imu.imu_py"
- Switch to find_packages() with package_dir for automatic package discovery
- Add saving/restoring of CWD in CMake build to prevent directory side effects

dm_imu.egg-info/SOURCES.txt

@@ -2,6 +2,7 @@ LICENSE
 README.md
 pyproject.toml
 setup.py
+./dm_imu/__init__.py
 dm_imu/__init__.py
 dm_imu.egg-info/PKG-INFO
 dm_imu.egg-info/SOURCES.txt

dm_imu.egg-info/top_level.txt

@@ -1,2 +1 @@
 dm_imu
-imu_py