extensionSystem: Prevent broken updates

Spidermonkey caches imports, which means that uninstalling an
old extension version and installing a new one doesn't work as
expected: If the previous version was loaded, then its code will
be imported instead.

For the last couple of releases this has been a reliable source
of extension bug reports after major GNOME updates. Thankfully
chrome-gnome-shell removed its update support in favor of our
built-in support now, but users may still use older versions
or perform those actions manually, so it still makes sense to
catch this case and set an appropriate error.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1248