using should trigger Pkg's parallel precompilation

[ericphanson]

I think Base should add a hook so that when Pkg is loaded, using PkgX triggers parallel precompilation instead of serial. It's not intuitive for that there's different precompilation modes with hugely different runtimes, and I think it's a common footgun to accidentally use the non-parallel version.

[Roger-luo]

Or can we have this by default, and have a flag to turn it off?

[IanButterworth]

I think to do this without regression Pkg.precompile needs to start showing the precompile warnings that are currently only shown in code load precompilation. Proposal here: JuliaLang/Pkg.jl#3319