Restore `$soucrce` in `WP_Upgrader::install_package()`.

See #22400.

Built from https://develop.svn.wordpress.org/trunk@28467


git-svn-id: http://core.svn.wordpress.org/trunk@28294 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Add inline comment after fix in [28467].

Props jesin.
Fixes #28292.


Built from https://develop.svn.wordpress.org/trunk@28468


git-svn-id: http://core.svn.wordpress.org/trunk@28295 1a063a9b-81f0-0310-95a4-ce76da25c4cd