Preprocess functions from base hooks not triggered for theme hooks not using the __ pattern

Problem/Motivation

A theme hook can specify a 'base hook' in order to inherit the preprocess functions.

This currently only works if the theme hook uses the __ pattern:

function mymodule_theme() {
  return ['foo__bar' => ['base hook' => 'foo',
    ],
  ];
}

Proposed resolution

Specifically handle the theme hooks not using a double underscore by merging directly, while leaving the while() loop handling in place for double underscores.

Remaining tasks

N/A

User interface changes

N/A

API changes

N/A

Data model changes

N/A