Create api for plugins to give deprecation warnings
Background
In the future we are planning on reducing the number of plug-ins in BuildStream core. Before we remove a plug-in, it should go through a period in which it's use displays a deprecation warning to the user. This warning should:
- Be individually silencable
- Inform the user where the to find the replacement for the deprecated plug-in
Task description
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Provide a way for plug-ins to provide deprecation warnings