The message is informative and should be visible to users running Composer.

Changed:
- Log informing user that the installer is running again with merge settings applied
- Visibility of `Logger::log()` method to output non-verbose messages

Adapted from `RequireCommand` in Composer 2.

In Composer v2, packages are solved first (dev and non-dev), then the lock file is written, then packages (dev and/or non-dev) are installed/updated/removed. As such, the closest alternative to the retired PRE_DEPENDENCIES_SOLVING is the new PRE_POOL_CREATE (and not PRE_OPERATIONS_EXEC as has been attempted in various plugins).

The use of PRE_DEPENDENCIES_SOLVING in the merge-plugin is used to inject duplicate requirements (usually with a different version constraints) into the solver (while distinguishing between require and require-dev).

To support Composer v1 and v2 without overcomplicating things, this commit replaces duplicate link tracking in `ExtraPackage` by back porting Composer 2's new static `MultiConstraint::create()` method which can be used to resolve complex-constraints early on. In turn, this makes the need for `PRE_DEPENDENCIES_SOLVING` obsolete.

Bug: T248908

Added:
- Allow either ^1.1 or ^2.0 composer-plugin-api
- New `PluginInterface` methods
- New `MultiConstraint` back port from Composer v2 for v1

Changed:
- Replaced `PluginState::$duplicateLinks` with `MultiConstraint` to merge requirements and constraints in `ExtraPackage::mergeOrDefer()` instead of during `PRE_DEPENDENCIES_SOLVING`
- Locking condition on `POST_PACKAGE_INSTALL` to force update in Composer v2
- Testing of `POST_PACKAGE_INSTALL` to reflect forced update in Composer v2
- Mocking of `VcsRepository` to support `HttpDownloader`  for Composer v2

Removed:
- Event `PRE_DEPENDENCIES_SOLVING` and method `MergePlugin::onDependencySolve()` since `ExtraPackage::mergeOrDefer()` handles duplicate package constraints
- Mocked event `PRE_DEPENDENCIES_SOLVING` in `MergePluginTest::triggerPlugin()` since duplicates are no longer tracked
- Assertions for obsolete `$extraInstalls` from `MergePluginTest::triggerPlugin()`

Added:
- "repositories.only" to limit the repository's scope

Changed:
- "repositories.type" to "path" to use the plugin from the parent directory

Added `Intervals::isSubsetOf()`, from composer/semver v3, to use the smallest possible constraint in `ExtraPackage::mergeConstraints()`.

Suggestion made by @Seldaek in: https://github.com/wikimedia/composer-merge-plugin/pull/189/files/a5c96e46520c6877792bc3d5fb01b44b0ab8d275#r511653048

Removed `—lock` from sub-level updating instructions.
Does not update sub-levels in Composer v1 or v2.

Added:
- Property and method `PluginState::isComposer1` to resolve Composer's version once

Changed:
- Class `MergePlugin` to use new `isComposer1` method
- Class `ExtraPackage` to use new `isComposer1` method

Fixed:
- Missing `PluginState` type-hint on `ExtraPackage::mergeOrDefer()`

Added:
- PHPUnit/PHPCS annotations to ignore file

Changed:
- Updated to composer/semver v3.2.4

Added:
- Instructions for safely upgrading the plugin to support Composer 2

Changed:
- Requirements to indicate Composer compatibility

Fixed:
- Syntax issues raised by PHPCS

Added:
- Action "ramsey/composer-install" to handle installation and caching
- Steps to test Composer v1 and Composer v2 as dependencies

Removed `InstallerEvent` and `InstallerEvents`.

Amends 4da0ee8

Fixed mix of yoda and non-yoda conditions.

Normalize naming across file.

Restore original behaviour to only trigger update when lock file is not locked.
In Composer 2, it is always locked.

Added console log to notify user they may need to perform a manual update.

Amends 4da0ee8

To prevent orphan version numbers when paired with software.

Adds tracking of merged requirements to `ExtraPackage` and `MergePlugin` to control if `MergePlugin::onPostInstallOrUpdate()` needs to trigger a `composer update` to install or update new requirements. The merged requirements are now assigned to `Installer::setUpdateAllowList()` to restrict the update operation to those new requirements. This prevents all other packages (such as root requirements) from being updated.

This solution provides consistent support of both the first-install feature and subsequent changes to merge-patterns between Composer 1 and 2.

Added:
- Property `ExtraPackage::$mergedRequirements` (and getter method) to track new requirements during merging process of `ExtraPackage::mergeOrDefer()`.
- Property `MergePlugin::$updateAllowList` (and getter method) to track all new requirements during merging process of `MergePlugin::mergeFile()`.
- Conditional statement in `MergePlugin::onPostInstallOrUpdate()` to skip first-install if there are no extra requirements.

Changed:
- Replaced `PluginState::forceUpdate()` conditional statement in `MergePlugin::onPostInstallOrUpdate()` with `Installer::setUpdateAllowList()`

FIxed:
- Property tags for `$loaded` and `$loadedNoDev` in `MergePlugin`

Fixes backwards compatibility with versions older than Composer 1.10.8.

Amends 4195d45