PTFM-8759 Eliminate import-time side effects from dxpy.current_job

* Remove top level attributes `dxpy.current_job`, `dxpy.current_app`,
  `dxpy.current_applet`
* Introduce accessor methods
  `dxpy.utils.exec_utils.get_current_job()`,
  `dxpy.utils.exec_utils.get_current_executable()`
* Add tests
* Add an autodoc section for `dxpy.utils` and submodules
* Update autodoc build dependencies

@kjlai please review

@psung interest