Install further packages automatically

PC has the notion of "dependent" or additional packages.  These are
defined in a standard "Package Control.sublime-settings" file, hosted
by the package.  (I.e. similar to the "dependencies.json".)

Installing such a package did not install the additional packages in one
go, neither did it notify about the incomplete install.  In the end, a
restart is enough and required to trigger `install_missing_packages()`
to catch up and install everything.

We have two options here.  (1) to bail out `install_package` with
`return None` and a message to tell the user to restart Sublime, or (2)
to install the packages in a recursive style.

Here we choose (1) which is a simple babystep for a better UX.  (2) is
not possible as all install side-effects need to run serialized.

Author: kaste

package_control/package_manager.py

@@ -1600,6 +1600,33 @@ def install_package(self, package_name, unattended=False):
                     fail_early=False
                 )
 
+            try:
+                extra_pc_settings_json = package_zip.read(common_folder + 'Package Control.sublime-settings')
+                extra_pc_settings = json.loads(extra_pc_settings_json.decode('utf-8'))
+            except (KeyError):
+                pass
+            except (ValueError):
+                console_write(
+                    '''
+                    Failed to parse the Package Control.sublime-settings for "%s"
+                    ''',
+                    package_name
+                )
+            else:
+                wanted_packages = set(extra_pc_settings.get('installed_packages') or [])
+                pc_settings = sublime.load_settings(pc_settings_filename())
+                in_process_packages = load_list_setting(pc_settings, 'in_process_packages')
+                additional_packages = wanted_packages - in_process_packages - self.installed_packages()
+                if additional_packages:
+                    console_write(
+                        '''
+                        Failed to install %s -
+                        deferring until next start to install additional packages
+                        ''',
+                        package_name
+                    )
+                    return None
+
             if package_name != old_package_name:
                 self.rename_package(old_package_name, package_name)