WIP: Do not skip dependencies when parts are given to the 'install' command

TODO: update tests and help message

src/zc/buildout/buildout.py

@@ -675,18 +675,16 @@ class Buildout(DictMixin):
         # From now, the caller will update the .installed.cfg at return.
         self.installed_part_options = installed_part_options
 
-        install_parts = self['buildout']['parts']
         if install_args:
             install_parts = install_args
             uninstall_missing = False
         else:
-            install_parts = install_parts.split()
+            install_parts = self['buildout']['parts'].split()
             uninstall_missing = True
 
         # load and initialize recipes
         [self[part]['recipe'] for part in install_parts]
-        if not install_args:
-            install_parts = self._parts
+        install_parts = self._parts
 
         if self._log_level < logging.DEBUG:
             sections = list(self)