Do not require jetpack_require_lib() to exist on the site

If `class WPCom_GHF_Markdown_Parser` already exists, then we can just
use it.

This allows site owners to integrate Markdown sync without requiring all
of Jetpack.  In this case they should copy the relevant code into their
site from here, and load it themselves:

https://github.com/Automattic/jetpack/tree/master/_inc/lib/markdown

As of this writing this code works independently of the rest of Jetpack.

wp-content/mu-plugins/markdown-sync/inc/class-importer.php

@@ -267,8 +267,14 @@ protected function update_post_from_markdown_source( $post_id ) {
 		if ( is_wp_error( $markdown_source ) ) {
 			return $markdown_source;
 		}
-		if ( ! function_exists( 'jetpack_require_lib' ) ) {
-			return new WP_Error( 'missing-jetpack-require-lib', 'jetpack_require_lib() is missing on system.' );
+		if (
+			! function_exists( 'jetpack_require_lib' ) &&
+			! class_exists( 'WPCom_GHF_Markdown_Parser' )
+		) {
+			return new WP_Error(
+				'missing-jetpack-require-lib',
+				'Missing `jetpack_require_lib()` or `class WPCom_GHF_Markdown_Parser`.'
+			);
 		}
 
 		// Transform GitHub repo HTML pages into their raw equivalents
@@ -330,7 +336,10 @@ protected function update_post_from_markdown_source( $post_id ) {
 		}
 
 		// Transform to HTML and save the post
-		jetpack_require_lib( 'markdown' );
+		if ( ! class_exists( 'WPCom_GHF_Markdown_Parser' ) ) {
+			// We checked for jetpack_require_lib() above.
+			jetpack_require_lib( 'markdown' );
+		}
 		$parser = new WPCom_GHF_Markdown_Parser();
 		$parser->preserve_shortcodes = false;
 		$html = $parser->transform( $markdown );