bpo-36146: Refactor setup.py: PyBuildExt.add() method (GH-12097)

* Add PyBuildExt.add() which adds the extension directly to
  self.extensions, rather than using a temporary 'exts' local
  variable in detect_modules() and then add 'exts' to self.extensions
* Convert 'missing' local variable from detect_modules()
  into PyBuildExt.missing attribute
* _detect_openssl(), _decimal_ext() and _detect_nis() now call
  directly self.add(), rather than returning an extension
  (or None if not found).
* Rename _decimal_ext() to _detect_decimal() for consistency with
  other methods.