[Data Liberation] Fork humanmade/WordPress-Importer (#1968)

This PR kicks off a streamable WXR importer that uses a PHP-only XML
parser, a PHP-only HTML parser, and a PHP-only streaming multi-request
HTTP client.

The first step is bringing in the `WXR_Importer` class from this
repository:

https://github.com/humanmade/WordPress-Importer

The importer is decoupled from the runtime and can run both in a request
handler and via CLI. It makes a great starting point for a streaming,
resumable, parallelized WXR importer that may eventually be proposed for
WordPress core.

Looping in the original plugin authors mentioned in the upstream README
file

Ryan Boren, Jon Cave (@joncave), Andrew Nacin (@nacin), Peter Westwood
(@westi), Ryan McCue (@rmmccue)

 ## Testing instructions

None, this PR only brings new code in. Everything else still works the
same.

packages/playground/data-liberation/phpcs.xml

@@ -1,6 +1,7 @@
 <ruleset name="WordPressStandard">
     <description>PHP 7.0 compatibility.</description>
     <config name="testVersion" value="7.2"/>
+    <exclude-pattern>src/WXR_Importer.php</exclude-pattern>
     <rule ref="PHPCompatibility">
         <exclude name="PHPCompatibility.Keywords.ForbiddenNamesAsDeclared"/>
     </rule>

packages/playground/data-liberation/src/WXR_Import_Info.php

@@ -0,0 +1,29 @@
+<?php
+/**
+ * A fork of the WordPress Importer plugin (humanmade/WordPress-Importer).
+ * It's meant to be rewritten to use the new libraries shipped with Data Liberation:
+ *
+ * * HTML parser
+ * * Streaming XML parser
+ * * Streaming multi-request HTTP client
+ *
+ * Original plugin created by Ryan Boren, Jon Cave (@joncave), Andrew Nacin (@nacin),
+ * and Peter Westwood (@westi). Redux project by Ryan McCue and contributors.
+ *
+ * See https://github.com/humanmade/WordPress-Importer/blob/master/class-wxr-importer.php
+ */
+class WXR_Import_Info {
+	public $home;
+	public $siteurl;
+
+	public $title;
+
+	public $users         = array();
+	public $post_count    = 0;
+	public $media_count   = 0;
+	public $comment_count = 0;
+	public $term_count    = 0;
+
+	public $generator = '';
+	public $version;
+}