feat(crossplane): split off core libs into separate repository

[Duologic]

The crossplane-libsonnet repository has grown quite big is struggling.

• The mkdocs site can't get rendered because the search index is too big
• jsonnet-bundler install takes a long time to download the GitHub archive
• the k8s CI job takes a lot of time, just a matter of time before that starts timing out on the free nodes

This only splits of a bit, follow up PRs needed to split off the others, after all providers are in separate repos, I supposse we can archive the original repo (keeping it available for people depending on it but not updating).