Fix undefined constant ANCHOR_LINK_LIBRARY_PATH by removing constants

The patch is incorrect, the reason it's mentioning undefined is because it's defined in the anchor_link.install file which is only loaded in certain circumstances.

I did a quick test and moving it to the module file is more likely to be available in more circumstances.

This will likely need a new release.

Also the file shouldn't be in the path name.

Also while debugging I noticed that

1. The path returned is the filesystem path with DRUPAL_ROOT, not the URL's path which failed to load
2. getLibraryPath gets called a few times so the result should be static cached

I've just ran into this same issue. The patch works.

But personally i think a constant on the AnchorLink class would be more appropriate. Then the constant doesn't get added to the global scope and is only called when necessary.

I agree, would you like to roll that patch or should I?

Here's the Class constant replacement @pookmish

Committed ... Thank you Joël for patching the better way!
Thanks, r81d3r for reporting and Mike for following up

Removing constants as it will bring issues on

[Thu Feb 20 10:34:50.607183 2020] [php7:notice] [pid 6103] [client ::1:39044] Error: Class 'Drupal\\anchor_link\\Plugin\\CKEditorPlugin\\AnchorLink' not found in /var/www/html/test/d8c7/web/modules/contrib/anchor_link/anchor_link.install on line 19 #0 [internal function]: anchor_link_requirements('install')\n#1 /var/www/html/test/d8c7/web/core/lib/Drupal/Core/Extension/ModuleHandler.php(392): call_user_func_array('anchor_link_req...', Array)\n#2 /var/www/html/test/d8c7/web/core/includes/install.inc(1048): Drupal\\Core\\Extension\\ModuleHandler->invoke('anchor_link', 'requirements', Array)

Trying to look at
https://git.drupalcode.org/project/drupal/commit/1edf15f

Planned to switch to use find from the LibrariesDirectoryFileFinder class
https://git.drupalcode.org/project/drupal/commit/1edf15f
When Drupal 8.9.x and Drupal 9.0.0 go out

But they still do have issues as getLibraryPath and getLibraryUrl. each will have it's used cases to check the phsical files and to call url link to images or + the origin Url -- removing language url ditection from the link to the image or js libraray.