CirrusSearch.php is not designed to be used in a migration script that will generate extension.php automatically. Several problems exist in this file that should be resolved first.

dynamic settings

$wgCirrusSearchIndexBaseName = wfWikiID();

These dynamic config var will be resolved at migration with a default value like:

"CirrusSearchIndexBaseName": {
        "value": "wikidb-mw_"
},

Unless there is a way to support "dynamic" with extension.json, we should remove this line from CirrusSearch.php and defer setting wgCirrusSearchIndexBaseName in the code when unset.

code/registry/factory setup
Sometimes with use php constants, these are also resolved by the migration script and become hard to read :

/**
 * Mapping of result types to CirrusSearch classes.
 */
$wgCirrusSearchFieldTypes = array(
	SearchIndexField::INDEX_TYPE_TEXT => \CirrusSearch\Search\TextIndexField::class,
	SearchIndexField::INDEX_TYPE_KEYWORD => \CirrusSearch\Search\KeywordIndexField::class,
[...]
);

will become:

          "CirrusSearchFieldTypes": {
			"value": {
				"0": "CirrusSearch\\Search\\TextIndexField",
				"1": "CirrusSearch\\Search\\KeywordIndexField",
[...]			}
		},

Maybe moving this into an extension hook would be appropriate.