Add dropzonejs composer merge

[andybroomfield]

See localgovdrupal/localgov#729

What does this change?

Adds the wikimedia composer merge plugin, and then uses that to load dropzonejs composer.libraries.json to require the dropzone JS library.

How to test

Composer install or composer update --lock should install the dropzonejs libary once that module is installed.

How can we measure success?

Module gets installed

Have we considered potential risks?

Others may want to have their own dropzone library, so will have to remove this.
Those who have created localgov drupal projects already will need to copy this step.

Images

n/a

Accessibility

n/a

[finnlewis]

Discussing in Merge Tuesday:

@stephen-cox leaning towards composer-merge plugin rather than the npm asset packagist

I suggest we test this again, get the tests running, then merge this to get things moving on localgovdrupal/localgov#729

Note: we'll need to add notes to the README.md and release notes to clarify this appraoch.

[andybroomfield]

This method does work, with a couple of caveats.
The first time the requirement for composer require drupal/simple_media_bulk_upload is run, this won't download the library as composer won't be aware of the merged libraries yet. Running composer update drupal/simple_media_bulk_upload -W (or dropzonejs) is required to get it pick up the merged plugins and download.
See localgovdrupal/localgov_assets#2

Also uninstalling / replacing does require a bit of composer.json surgery to remove it first before adding otherwise a enyo/dropzone can not be found error occurs.

[finnlewis]

I think we should go with this!

Existing sites will have someone managing the composer.json and can make the call on how to include libraries, following release notes in some cases.

For new installs, they will get a good starting point.

[finnlewis]

@andybroomfield are we going with it like this?

Shall we just merge it?

[finnlewis]

@willguv this is the one we're pushing for! With appropriate release notes.