[feature] Merge all PopGen_.jl packages under PopGen.jl monorepo

[pdimens]

The goal is to make PopGen.jl a monorepo like Makie

Benefits:

1. One repository, obviously. Currently, there is PopGen, PopGenCore and PopGenSims, the last of which lives as a repo under my personal account.
2. It might make CI easier, since everything depends on PopGenSims, and upstream <-> downstream testing would be super helpful.