Allow jQuery Validation library to be installed outside of module folder

Problem/Motivation

This module depends on the external jQuery Validation library. This library cannot be committed to the Clientside Validation module repository because:

• Drupal.org has a strict GPLv2+ policy for the drupal.org contributions repository, which conflicts with certain libraries.
• Modules and themes on drupal.org and their release cycles should not be affected by the release cycles of external libraries.

Currently, the Clientside Validation module's README recommends installing the jQuery Validation library by "extract[ing] the zip file to the js/lib of this sub module". However, is not recommended because:

• Best practices for module upgrades dictate to delete the old module folder before the upgrade. This removes any libraries inside of that folder.
• Multiple modules can implement the same library.
• In this particular module, a .gitignore file makes it difficult to commit the jQuery Validation library to the project, which makes it difficult for site builders who use Git to manage files on a website, such as on Acquia Cloud and Pantheon, and some self-hosted setups.

Drupal core / the official Drupal 8 documentation does not specify where modules should be placed. However, it seems to have become convention (see the modules: leaflet, slick, blazy) to locate libraries relative to the Drupal root at /libraries/$library_machine_name. Note, however, this requires a change to the path specified for the library in the module's *.libraries.yml file: paths to library files must begin with /libraries/$library_machine_name/.

For more information, please refer to the *.libraries.yml files of the aforementioned contributed modules, and the documentation for LibraryDiscoveryParser::parseLibraryInfo().

Proposed resolution

Update the jquery.validate definition in clientside_validation_jquery/clientside_validation_jquery.libraries.yml to look like:

jquery.validate:
  js:
    /libraries/jquery-validation/dist/jquery.validate.js: {}

Modify the README to specify where to put the library. See the Leaflet module's README for inspiration.

Update the .gitignore file at the root of the project, and delete the clientside_validation_jquery/js/lib/.gitkeep file.

Remaining tasks

1. Write a patch
2. Feedback and review
3. Feedback and RTBC
4. Commit and release
5. Update project page to specify where to put the library

User interface changes

None.

API changes

None.

However, it does change where the module expects the library to be, so it will break people's existing installations. This will almost certainly warrant a note in the module release notes, but there doesn't seem to be a convention on whether this would also warrant a new major or minor version number, a new hook_update_N() that displays a message, a message in the status report if the library is detected at its old location, a message displayed to administrators on every page load until the problem is fixed, or some combination of these. Feedback on the module maintainers' preference would be appreciated so that could be added to the patch!

Data model changes

None.