Replace our homegrown plugin system with pluggy

[thilp]

Is your feature request related to a problem? Please describe.

As a maintainer of Transformer, the less code I have to maintain, the better. This is particularly true of code that is not immediately relevant to Transformer's main goal, like the "plumbing" part of its plugin system.

Take for example:

• transformer.plugins.contracts,
• transformer.plugins.resolve,
• their unit tests.

Taken together, they make "just" around 400 lines of code, but that's 10% of all Transformer, and that's not counting the corresponding user documentation (no good tools to count that quickly).

Describe the solution you'd like

I would like this "plumbing" to be taken care of by a third-party library like pluggy (thank you @marcinzaremba for the recommendation), which has battle-tested its design at a much larger scale than Transformer. I don't want to solve the same problems in an unrelated project.

It doesn't have to be pluggy, but at least it shouldn't be done in Transformer (and it should be done well enough that not doing it in Transformer simplifies our life, not the opposite).

Describe alternatives you've considered

…

Additional context

We should decide before many users start writing their own Transformer plugins, so that we can have a simple deprecation process for the current plugin system. I don't think we want to be maintaining two plugin systems in parallel.