Disallow Pkg.precompile during precompilation

[IanButterworth]

Proposed fix for #3458

Basically make it noop during precompilation, and noisy if it's specifically called, silent if it's autoprecompilation.

But I'm not sure if this would prevent Pkg.precompile being baked into the julia sysimage, and I'm pretty sure it will stop it from being baked into a Pkg pkgimage.

@vchuravy

[vchuravy]

Having precompile statements should still work fine.

[KristofferC]

We call this while creating the sysimage and also want to call it during precompilation when Pkg is not in sysimage.

[IanButterworth]

I thought @vchuravy's comment meant that when julia is in the mode to generate precompile statements jl_generating_output == 0? So the sysimage mechanism should still work?

As for pkgimage precompilation, that does seem like it would need to circumvent this, which could be put in place as an internal kwarg that could be set, perhaps?

[vchuravy]

How do we prevent the fork-bomb from occuring if we were to execute this during the precompilation of Pkg? Probably because we changed to a temporary project?

For PKG, the old method of collecting precompilation statements and then including those and regenerating the cache file might be better?