Extensions - Setup <classloader> during "enable" and "uninstall"

Extensions - Setup <classloader> during "enable" and "uninstall"

This is an analog/follow-up to https://github.com/civicrm/civicrm-core/pull/20091

Why is this necessary? Recall that a typical admin will go through this lifecycle:

1. Enable extension $x
2. Disable extension $x
3. Either:
   * (a) Re-enable extension $x
   * (b) Uninstall extension $x

Step `#2` disables the classloader for purposes of regular page-loading. However, when you
get to step `#3a` or `#3b`, then you need the classloader again.