refactor the plugin to make integration-tests possible

currently the code of `CycloneDX\Composer\Plugin` is not integration-testable in a good way,
and needs some love.

since the whole code is marked as `@internal` a refactoring can be done without causing breaking changes.

Start by reading the `CycloneDX\Composer\Plugin` class and inspect how its current structure prevents integration testing. Identify the refactoring scope and define integration tests that exercise the plugin; done means those tests can run against the refactored design without breaking the existing internal API.

Written by the indexing model from the issue text.