External Libraries: Update Requests library to version 2.0.0.

This is a major release and contains breaking changes.

Most important changes to be aware of for this release:
* All code is now namespaced. Though there is a full backward compatibility layer available and the old class names are still supported, using them will generate a deprecation notice (which can be silenced by plugins if they'd need to support multiple WP versions). See the [https://requests.ryanmccue.info/docs/upgrading.html upgrade guide] for more details.
* A lot of classes have been marked `final`. This should generally not affect userland code as care has been taken to not apply the `final` keyword to classes which are known to be extended in userland code.
* Extensive input validation has been added to Requests. When Requests is used as documented though, this will be unnoticable.
* A new `WpOrg\Requests\Requests::has_capabilities()` method has been introduced which can be used to address #37708.
* A new `WpOrg\Requests\Response::decode_body()` method has been introduced which may be usable to simplify some of the WP native wrapper code.
* Remaining PHP 8.0 compatibility fixed (support for named parameters).
* PHP 8.1 compatibility.

Release notes: https://github.com/WordPress/Requests/releases/tag/v2.0.0

For a full list of changes in this update, see the Requests GitHub:
https://github.com/WordPress/Requests/compare/v1.8.1...v2.0.0

This commit also resolves 2 blocking issues which previously caused the revert of [52244]:

* New Requests files are loaded into `wp-includes/Requests/src/`, matching the location of the library. In doing so, filesystems that are case-insensitive are not impacted (see #54582).
* Preload: During a Core update, the old Requests files are preloaded into memory before the update deletes the files. Preloading avoids fatal errors noted in #54562. 

Follow-up to [50842], [51078], [52244], [52315], [52327], [52328].

Props jrf, schlessera, datagutten, wojsmol, dustinrue, soulseekah, szepeviktor. costdev, sergeybiryukov, peterwilsoncc, ironprogrammer, antonvlasenko, hellofromTonya, swissspidy, dd32, azaozz, TobiasBg, audrasjb.
Fixes #54504.
See #54582, #54562.

git-svn-id: https://develop.svn.wordpress.org/trunk@54997 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -976,7 +976,7 @@ function do_enclose( $content, $post ) {
  *
  * @param string $url        URL to retrieve HTTP headers from.
  * @param bool   $deprecated Not Used.
- * @return \Requests_Utility_CaseInsensitiveDictionary|false Headers on success, false on failure.
+ * @return \WpOrg\Requests\Utility\CaseInsensitiveDictionary|false Headers on success, false on failure.
  */
 function wp_get_http_headers( $url, $deprecated = false ) {
 	if ( ! empty( $deprecated ) ) {