composer in packages.json causes dependencies installation error

With composer packages in a plugin, installing dependencies causes object/array error:

Same error, when opening check package.

json_decode should be displayed as array, not object as surely intended. Next line is foreach ($datas['installed'] not foreach ($datas->installed

Here is the fix.

Description

Fixes #

[x] Bug fix (non-breaking change which fixes)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)
- [ ] This change is only breaking for integrators, not for external standards or end-users.
[ ] Documentation improvement

[ ] I have checked there is no other PR open for the same change.
[ ] I have read the [La ligne directrice pour contribuer à ce projet / Contribution guidelines for this project).
[ ] I grant the project the right to include and distribute the code under the GNU.
[ ] I have added tests to cover my changes.
[ ] I have verified that the code complies with the projects coding standards.
[ ] [Required for new sniffs] I have added MD documentation for the sniff.