Read extension.json files in the web updater

Read extension.json files in the web updater

The web updater reads LocalSettings.php in function scope to figure out
what extensions have been loaded. This doesn't work for extensions being
loaded through the ExtensionRegistry since they're only added to a
queue.

This adds the ability for the installer to read and get information from
the current extension load queue. LocalSettings.php is still read for
extensions that have not been converted yet.

Other uses of Installer::getExistingLocalSettings() were audited and
determined to be safe with regards to extension usage.

Extensions that register hooks using explicit globals
($GLOBALS['wgHooks']['FooBar']) are still broken.

Bug: T100414
Change-Id: Icc574a38a7947a1e3aff8622a4889e9dcfd7a4b2