Add support for composer/installers v1 and v2

[mcaskill]

Context

Provide support for composer/installers versions 1 and 2, for peer dependencies that might require the latter.

Summary

This PR can be summarized in the following changelog entry:

• [non-user-facing] Provide support for composer/installers versions 1 and 2.

Relevant technical choices:

• Update version constraint on composer/installers dependency to support:
  • >=1.12.0 <2.0.0-0 or
  • >=2.0.0 <3.0.0-0

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

1. Install WordPress plugin using Composer versions 1.10, 2.1, 2.2, 2.3, or later.
2. Validate that the plugin is installed in the expected directory as defined by the install path for the wordpress-plugin type.

Test instructions for QA when the code is in the RC

• [x] QA should use the same steps as above.

Impact check

This PR affects the following parts of the plugin, which may require extra testing:

• Installation using Composer, dependencies that explicitly require composer/installers version 1 or 2.

UI changes

• [ ] This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Other environments

• [ ] This PR also affects Shopify. I have added a changelog entry starting with [shopify-seo], added test instructions for Shopify and attached the Shopify label to this PR.

Documentation

• [ ] I have written documentation for this change.

Quality assurance

• [x] I have tested this code to the best of my abilities
• [ ] I have added unittests to verify the code works as intended
• [ ] If any part of the code is behind a feature flag, my test instructions also cover cases where the feature flag is switched off.
• [x] I have written this PR in accordance with my team's definition of done.

Fixes #18486

[private-packagist[bot]]

composer.lock

Package changes

Package	Operation	From	To	Changes
composer/installers	upgrade	v1.12.0	v2.1.1	diff

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[mcaskill]

I've rebased the trunk branch to 0d97da21

[mcaskill]

I've rebased the trunk branch to 957b5f62

[mcaskill]

I've rebased the trunk branch to dc921e3c

[herregroen]

@mcaskill Thank you very much for the PR!

There's some tests currently still failing so I'm taking a look now to see what we can do to ensure those pass.

Our PHP 5.6 test runs have a few odd steps that need adjustment to account for this. I'll be looking into those and will merge this PR after that.

[herregroen]

I've just merged https://github.com/Yoast/wordpress-seo/pull/18871 which includes fixed PHP 5.6 builds but is otherwise the same.

Thank you again for your contribution @mcaskill! Props have been given in the replacement PR!