Create a separate coq-lithium package

Merged Michael Sammler requested to merge ci/multiple-packages-2 into master

This MR creates a separate coq-lithium packages such that one can install Lithium without needing to pull in the full RefinedC (especially without needing to compile cerberus). Sadly, we need a hack to remove lithium from the refinedc dune files before installing, since otherwise dune gets confused.

@lepigre any thoughts on this?

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