Register declaratively and remove the root entry-point class

[alistair3149]

What

Completes the move of this extension's registration into extension.json, eliminating the legacy procedural entry point. Settings were already declarative (config + config_prefix: seqlg); this change removes the last procedural piece — the root SemanticExternalQueryLookup class and its ExtensionFunctions callback.

Why an extension function is kept (not fully declarative)

The callback set up two class_alias calls — SMWExternalAskQueryLookup (the documented public name used in $smwgQuerySources) and the deprecated SMWExternalQueryLookup. class_alias has no declarative equivalent in extension.json and must run at setup time, so a thin extension function is unavoidable here. It is now a namespaced SEQL\Setup::onExtensionFunction rather than a root-class static method.

Changes

• Add SEQL\Setup holding the class_alias calls and the HookRegistry registration, reading seqlgExternalRepositoryEndpoints directly from the populated config global.
• extension.json: point ExtensionFunctions at SEQL\Setup::onExtensionFunction; drop AutoloadClasses (the class now autoloads via AutoloadNamespaces).
• Drop load_composer_autoloader: there is no composer autoload block and no third-party runtime dependency (require is php + composer/installers only — the mediawiki/http-request runtime dep was already removed), so there is nothing local to load.
• Drop the SEQL_VERSION constant and SemanticExternalQueryLookup::getVersion(); the test bootstrap now reads the version from extension.json.
• Delete the root SemanticExternalQueryLookup.php.

No user-facing configuration changes: the seqlg* globals, the SMWExternalAskQueryLookup/SMWExternalQueryLookup aliases, and all hook behavior are preserved.

Verification

Run in the docker-compose-ci stack against MW 1.43 / SMW dev-master:

• composer analyze clean (parallel-lint + PHPCS + minus-x, no warnings).
• composer phpunit green (101 tests, 194 assertions).
• Live checks via eval.php: seqlg* config globals populated; InterwikiLoadPrefix, smwAskParserFunction, smwShowParserFunction registered in the hook container; SEQL\Setup present; root SemanticExternalQueryLookup class gone; both SMWExternal* aliases resolve to SEQL\ByHttpRequestQueryLookup; SEQL_VERSION no longer defined.
• Live render path: #ask parses with no error/exception markup, and invoking the registered smwAskParserFunction handler through the hook container correctly gates the source= parameter by namespace — confirming the relocated callback wires a working handler end-to-end.