Plugins: When loading a plugin in a "sandbox" on activation, do it in a separate function.

This avoids accidentally overriding some variables in the scope of `activate_plugin()`, e.g. `$silent` or `$network_wide`.

Plugins expecting to have access to `$network_wide` directly on inclusion should receive it as an argument of the activation hook callback instead, on any of these actions:

* `activate_plugin`
* `activate_{$plugin}`
* `activated_plugin`

Follow-up to [28644].

Props Mike_Cowobo, dd32, DrewAPicture, mensmaximus, SergeyBiryukov.
Fixes #31104.
Built from https://develop.svn.wordpress.org/trunk@50787


git-svn-id: http://core.svn.wordpress.org/trunk@50396 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.8-alpha-50786';
+$wp_version = '5.8-alpha-50787';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.