template-loader does not check for child theme's index.php

[koopersmith]

When template-loader reaches the last case in the if statement, it checks for only the parent theme's index.php. Template-loader should first check for the child's index.php, then the parent's index.php. It should use locate_template, which will check for both the parent and the child and behave accordingly.

To replicate the bug, make a parent and child theme each with only index.php. In no instances should the parent's index.php be shown, but on archive pages (and others) it will appear.

For easier bug replication:

1. Make a child theme of twentyten with a blank index.php
   
2. Delete twentyten's archive.php
   
3. Navigate to an archive page

Interesting thought. Should probably introduce get_index_template() which returns get_query_template('index').

Introduce get_index_template()

(In [13819]) Introduce get_index_template(). Allows child themes to override a parent theme's index.php. Props koopersmith, fixes #12635