Conditionally adapt to Core Requests library

[schlessera]

This PR changes the logic to integrate with Requests so that it conditionally adapts to the version that ships with WordPress Core or falls back to the bundled one as needed.

[danielbachhuber]

As a FYI, I'm getting this error in wp-cli-dev:

$ wp maintenance release generate entity-command

Fatal error: Uncaught Error: Class "WpOrg\Requests\Autoload" not found in /Users/danielbachhuber/projects/wp-cli-dev/wp-cli/php/WP_CLI/Bootstrap/IncludeRequestsAutoloader.php:83
Stack trace:
#0 /Users/danielbachhuber/projects/wp-cli-dev/wp-cli/php/bootstrap.php(83): WP_CLI\Bootstrap\IncludeRequestsAutoloader->process(Object(WP_CLI\Bootstrap\BootstrapState))
#1 /Users/danielbachhuber/projects/wp-cli-dev/wp-cli/php/wp-cli.php(32): WP_CLI\bootstrap()
#2 /Users/danielbachhuber/projects/wp-cli-dev/wp-cli/php/boot-fs.php(17): require_once('/Users/danielba...')
#3 {main}
  thrown in /Users/danielbachhuber/projects/wp-cli-dev/wp-cli/php/WP_CLI/Bootstrap/IncludeRequestsAutoloader.php on line 83

[danielbachhuber]

As a FYI, I'm getting this error in wp-cli-dev

Deleting wp-cli-dev/composer.lock and running composer install fixed the issue for me.

[jmtorres]

This doesn't seem to resolve the case when connecting to a remote WP site through SSH with an alias (and probably by other means). I submitted another issue in relation to this: #5816