Most Drupal core files now live in a "core" subdirectory

All Drupal core modules, themes, scripts, libraries, includes, and text files have been moved to a "/core" subdirectory of the Drupal repository with the exception of these files/directories:

• profiles
• sites
• .htaccess
• example.gitignore
• authorize.php
• index.php
• install.php
• README.txt
• robots.txt
• web.config

Placement of non-core modules and themes

• Following this change, contributed and custom modules can now be placed in the modules/ folder in the root of the Drupal installation, and contributed and custom themes can be placed in the similar themes/ folder.
• This differs from Drupal 7 and earlier, where these folders were used for core modules and themes.
• The sites/all/modules/ and sites/all/themes/ folders can also still be used, as can site-specific subdirectories.
• The core/modules/ and core/themes/ directories should never be used for contributed or custom modules.