[Dev] import vs. using

@ocots minor point: we currently import CTX inside OC.jl according to import CTX: CTX, foo... (not bar)
- CTX packages don't export anything
- prefixing is nice (CTX.bar), so this is fine to avoid ambiguities
- since we usually do not *redefine* CTX.foo, using CTX: CTX, foo should be more coherent

OK with that?

i think the clear rule is:
- CTX don't export
- using / import CTX: CTX, foo # not bar
- foo since we want to (re)export it from OC (so that the user has it when using OC)
- bar is not exported
- with using and not import, unless we redefine foo