Support "extra" dependencies/non-declared transitive dependencies

[tgolsson]

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

Some packages have optional dependencies that aren't properly declared in their metadata. For example, the torch module torch.utils.tensorboard needs tensorboard. It may also sometimes need Pillow to write images to log files. Similarly, the transformers library can use bitsandbytes for optimizations but does not have any extras or dependencies for it.

This leads to issues with dependency inference, since a Pants user would expect all transitive dependencies to be included. The solution for this is to declare explicit dependencies on these packages. However, since pants already ships a list for mapping modules to names, it could be interesting to also support (and ship) a module-to-dependencies list in the same way.

Describe the solution you'd like

One possible way of declaring this would be a system similar to module_mappings.

# BUILD
python_requirements(
    ...,
    undeclared_dependencies={
        "torch.utils.tensorboard": ["tensorboard", "Pillow"],
        "transformers": ["bitsandbytes"],
    }
)

Describe alternatives you've considered
It is possible to solve this with explicit dependencies.

Additional context

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