BookManager::buildItems() is slow because it loads nodes

The only reason the node is loaded is in order to get $node->urlInfo(). In my limited testing, "entity:node/$nid" works just as well. (This is passed to Url::fromUri().) The attached patch makes this little change.

Originally, I counted using array_walk_recursive() on the tree of links returned by bookTreeAllData(). I came up with a much larger number, since each link it itself a structured array. I came up with 3,600 by counting the number of <a> elements in the HTML markup for the navigation.

I am not sure of my analysis. I originally thought that Node::loadMultiple() was called indirectly by bookTreeAllData(). That may be correct, but now I cannot find such a call to confirm what I said. I may have been looking at a call to BookOutlineStorageI::loadMultiple().

I am pretty sure that I need both the optimization in this issue and some additional caching in order to improve my page-load times. I tried using each by itself and did not find any improvement.

Those links are tested in Drupal\Tests\book\Functional\BookTest::testBook(). It is a little hard to follow, especially since that test uses methods from Drupal\Tests\book\Functional\BookTestTrait. So why not hack the patch and let the testbot show us where the links are tested?

I am setting the status back to RTBC and re-uploading the patch from #3 so that the testbot does not get confused.

Unfortunately, the testbot seems to be truncating the error messages :-(. Here is what I get when I run testBook() locally:

$ lando phpunit -c /app/core core/modules/book/tests/src/Functional/BookTest.php 
PHPUnit 6.5.14 by Sebastian Bergmann and contributors.

Testing Drupal\Tests\book\Functional\BookTest
..E............                                                   15 / 15 (100%)

Time: 8.97 minutes, Memory: 6.00MB

There was 1 error:

1) Drupal\Tests\book\Functional\BookTest::testBook
Behat\Mink\Exception\ExpectationException: The pattern /<nav id="book-navigation-1"(.*?)<ul(.*?)(node\/2)(.*?)(01 - SimpleTest test node kbaSQdvZ2I)(.*?)(node\/5)(.*?)(04 - SimpleTest test node xLfEnr8N3e)(.*?)(node\/6)(.*?)(05 - SimpleTest test node yaQHvKxGs2)(.*?)<\/ul>/s was not found anywhere in the HTML response of the page.

/app/vendor/behat/mink/src/WebAssert.php:768
/app/vendor/behat/mink/src/WebAssert.php:354
/app/core/tests/Drupal/FunctionalTests/AssertLegacyTrait.php:690
/app/core/modules/book/tests/src/Functional/BookTestTrait.php:98
/app/core/modules/book/tests/src/Functional/BookTest.php:137

ERRORS!
Tests: 15, Assertions: 619, Errors: 1.

HTML output was generated
https://drupal.lndo.site/sites/simpletest/browser_output/Drupal_Tests_book_Functional_BookTest-575-96303946.html
...

@joachim, @alexpott:

Thanks for taking a look at this issue! It is nice to know that, even during the mad dash to get 9.0 ready for beta, we can still get little fixes like this one into the codebase.