Clarify HAVE_PYTHON_APT/install_python_apt handling in apt_repository

1. Don't test check_mode in both the caller and in the callee.
2. Don't test HAVE_PYTHON_APT inside an if that tests HAVE_PYTHON_APT
3. Don't be irritatingly vague about why the module fails ("You may be
   seeing this because…").

Note that if «apt-get -y install python-apt» succeeds with rc==0, but
for some reason python_apt is not usable afterwards, this will break
because the imports in install_python_apt aren't wrapped inside a
try/except.

In other words, we assume that install_python_apt either succeeds or
fails with a traceback. This commit doesn't affect that behaviour.

lib/ansible/modules/packaging/os/apt_repository.py

@@ -423,24 +423,24 @@ def main():
     )
 
     params = module.params
-    if params['install_python_apt'] and not HAVE_PYTHON_APT and not module.check_mode:
-        install_python_apt(module)
-
     repo = module.params['repo']
     state = module.params['state']
     update_cache = module.params['update_cache']
     sourceslist = None
 
-    if HAVE_PYTHON_APT:
+    if not HAVE_PYTHON_APT:
+        if params['install_python_apt']:
+            install_python_apt(module)
+        else:
+            module.fail_json(msg='python-apt is not installed, and install_python_apt is False')
+
     if isinstance(distro, aptsources_distro.UbuntuDistribution):
         sourceslist = UbuntuSourcesList(module,
             add_ppa_signing_keys_callback=get_add_ppa_signing_key_callback(module))
-        elif HAVE_PYTHON_APT and \
+    elif isinstance(distro, aptsources_distro.DebianDistribution) or isinstance(distro, aptsources_distro.Distribution):
-            isinstance(distro, aptsources_distro.DebianDistribution) or isinstance(distro, aptsources_distro.Distribution):
         sourceslist = SourcesList()
     else:
-        module.fail_json(msg='Module apt_repository supports only Debian and Ubuntu. ' + \
+        module.fail_json(msg='Module apt_repository supports only Debian and Ubuntu.')
-                             'You may be seeing this because python-apt is not installed, but you requested that it not be auto-installed')
 
     sources_before = sourceslist.dump()