AVRO-4018: [php] Replace monorepo plugin with simpler configuration

[thiagorb]

What is the purpose of the change

This PR removes the need for the dependency on the composer plugin (https://github.com/beberlei/composer-monorepo-plugin).

With these changes, to use the PHP library, one can just add the repo to composer configuration:

composer config repositories.repo-name vcs https://github.com/apache/avro

And then install it without problems:

composer require apache/avro:dev-main

or (when a tag gets created) using a version:

composer require apache/avro:x.y.z

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable