Skip predefined meta keys in WXR import

[fredrikekelund]

The canonical WordPress Importer skips a few predefined post meta keys (_wp_attached_file, _wp_attachment_metadata, and _edit_lock). The php-toolkit implementation doesn't, which breaks attachment URLs during WXR imports.

This PR changes that. It applies the equivalent logic to the canonical WordPress Importer to skip over those predefined meta keys.

Testing instructions

1. Have Studio installed
2. Create a new site in Studio
3. Quit Studio
4. Run ln -s SITEDIR/wp-content/mu-plugins/sqlite-database-integration/ SITEDIR/wp-content/plugins/sqlite-database-integration
5. Download this blueprint bundle: http://blueprintlibrary.wordpress.com/?blueprint=829
6. Extract it
7. Run composer build-blueprints-phar
8. Run php dist/blueprints.phar exec PATH_TO_BLUEPRINT_JSON --mode=apply-to-existing-site --site-path=PATH_TO_SITE --site-url=http://localhost:PORT --db-engine=sqlite (where PATH_TO_BLUEPRINT_JSON is the path to the blueprint.json file in the blueprint bundle you extracted before, PATH_TO_SITE is the path to the site you created in Studio, and PORT is the port of the site)

[fredrikekelund]

The changes in this file are not part of the core purpose of this PR. However, blueprints.phar failed when the PHP opening and closing tags were present, which seems to make sense, as this content is concatenated with components/Blueprints/Steps/scripts/import-content.php (which doesn't have a closing PHP tag)

[fredrikekelund]

@adamziel @ashfame I'd appreciate a review here when you get a chance 🙏

The failing CI run seems like a fluke. Should hopefully pass if you retrigger it