bpo-40173: Fix test.support.import_helper.import_fresh_module() (GH-28654)

* Work correctly if an additional fresh module imports other
  additional fresh module which imports a blocked module.
* Raises ImportError if the specified module cannot be imported
  while all additional fresh modules are successfully imported.
* Support blocking packages.
* Always restore the import state of fresh and blocked modules
  and their submodules.
* Fix test_decimal and test_xml_etree which depended on an undesired
  side effect of import_fresh_module().