Replace curl-based HTTP client with WordPress\AsyncHttp\Client

[adamziel]

Make the URL rewriter download WordPress assets without curl dependency.

Benefits:

• No dependencies = this can eventually become a part of WordPress core
• The code is much simpler
• We'll get more testing for WordPress\AsyncHttp\Client and will end up building less curl-specific code

The implementation is pretty much this:

$client = new Client( [
	'concurrency' => 10,
] );
$client->enqueue( $requests );

$results = [];
while ( $client->await_next_event() ) {
	$request = $client->get_request();
	
	switch ( $client->get_event() ) {
		case Client::EVENT_BODY_CHUNK_AVAILABLE:
			file_put_contents(
				$local_paths[$request->original_request()->id],
				$client->get_response_body_chunk(),
				FILE_APPEND
			);
			break;
		case Client::EVENT_FAILED:
			$results[$request->original_request()->url] = [
				'success' => false,
				'error' => $request->error,
			];
			break;
		case Client::EVENT_FINISHED:
			$results[$request->original_request()->url] = [
				'success' => true
			];
			break;
	}
}

To test, run

$ php bin/rewrite-urls.php migrate_urls \
    --file ./adamadam-blog.html \
    --current-site-url https://adamadam.blog \
    --new-site-url https://adamziel.com

And you should see a new assets directory with bunch of static assets fetched from adamadam.blog in it.

See WordPress/blueprints-library#113 for more details about the HTTP client library.

Follow-up work

• Expand to stream-rewriting remote HTML pages
• Expand to stream-rewriting remote Zipped HTML pages (with ZipStreamReader).
• Expand to stream-rewriting remote WXR files (with rewrite-wxr.php I'm exploring for the Blueprints community space)
• Expand to stream-rewriting remote zipped WXR files (combining the two techniques above)

cc @dmsnell @akirk @brandonpayton @bgrgicak @griffbrad