Allow dependency management via asset-packagist

Problem/Motivation

Requiring the library via asset-packagist stores it in
/libraries/drupal--autocomplete/
with the build files available under
/libraries/drupal--autocomplete/dist/

The current _a11y_autocomplete_element_find_library() doesn't find this location.

Steps to reproduce

* Add asset-packagist as a repository
* Add "vendor:npm-asset" to the "web/libraries/{$name}" installer-paths.
* composer require npm-asset/drupal--autocomplete

Proposed resolution

Add the missing detection to _a11y_autocomplete_element_find_library().

Remaining tasks

n/a

User interface changes

n/a

API changes

n/a

Data model changes

n/a