Use dependency injection for plugin dependencies

@pvbergen
Fixed all phpcs issues except this one -
FILE: /pagedesigner/tests/src/Kernel/AbstractPagedesignerTest.php
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FOUND 1 ERROR AFFECTING 1 LINE
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21 | ERROR | Do not disable strict config schema checking in tests. Instead ensure your module properly declares its schema for configurations.
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Please review and let me know in case of any concerns.

Thanks for raising this issue. This is already being resolved in 4.x, which also will include some interface changes.
We won't update 3.x with these changes anymore.

Closing as outdated.