Coding Standards: Move wp-includes/class-http.php to `wp-includes/c…

…lass-wp-http.php`.

This renames the file containing the `WP_Http` class to conform to the coding standards.

This commit also includes:

- A new `class-http.php` that includes the new file, for anyone that may've been including the file directly.
- Replaces references to the old filename with the new filename.

Follow-up to [8516], [13274], [33748].

Fixes #54389. See #53359.
Built from https://develop.svn.wordpress.org/trunk@52026


git-svn-id: http://core.svn.wordpress.org/trunk@51618 1a063a9b-81f0-0310-95a4-ce76da25c4cd