Update python install + build-tree setup (#128)

- When one python version is used, install to proper lib/pythonX.Y/site-packages
- config files, etc. in build tree resemble the install-tree

tests/CMakeLists.txt

@@ -106,7 +106,7 @@ set(_python_environment
     "OMNITRACE_USE_PID=OFF"
     "OMNITRACE_TIMEMORY_COMPONENTS=wall_clock,trip_count"
     "${_test_library_path}"
-    "PYTHONPATH=${PROJECT_BINARY_DIR}/lib/python/site-packages")
+    "PYTHONPATH=${PROJECT_BINARY_DIR}/${CMAKE_INSTALL_PYTHONDIR}")
 
 set(_attach_environment
     "OMNITRACE_USE_PERFETTO=ON"