JLLExtras.jl package

[staticfloat]

We should create a JLLExtras.jl package that allows for meta-inspection/management of JLLs. This package doesn't have to be as hyper-optimized as JLLWrappers so we can use it to dev out a JLL for editing, etc...

Examples of things we want to be able to do:

• collect_artifacts(jll_name::String; recursive::Bool, extra_artifacts::Vector{String}) Extract the artifacts of the given jll_name (as looked up from the current pkg environment) out to a scratch space for building/linking against. This should replace the current dev_jll() implementation, and notably should use Preferences and Scratch. We can dump the whole idea of the overrides directory now that Preferences is working properly. recursive will determine whether we will extract all dependencies into the same directory (useful for people who just want to link against a tree of dependencies with minimum hassle) and extra_artifacts will be used to merge together multiple artifacts, given that Mosè and I ever get around to splitting things up into runtime, debug, logs, etc... artifacts.