Move parallel precompile out of Pkg

[KristofferC]

Parallel precompile is pretty much independent of Pkg. As Pkg is getting moved out of the sysimage it becomes awkward to have it there. For example, you still want parallel precompile for package load even if you haven't loaded Pkg in the session. And issues like JuliaLang/Pkg.jl#3442 also shows it is awkward to have to load Pkg in various environment just to get access to the precompilation feature.

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1. precompilation: capture stdout as well as stderr Pkg.jl#3573
   precompile +1
   
2. Pkg fails to print all output from subprocesses Pkg.jl#3488
   precompile +1
3. stdout messages lost by precompile [regression] Pkg.jl#3548
   precompile +1
   
4. show output for explicitly precompiled packages immediately Pkg.jl#3487
   precompile +1
5. Unhandled InterruptException during precompilation Pkg.jl#2648
   bug precompile +2
6. Windows formatting terminal characters error Pkg.jl#3068
   precompile windows +2
7. precompile: if a single package is requested, show output live Pkg.jl#3582
   precompile +1
   
8. Pkg.precompile only aware of extensions that should be loaded due to current Manifest Pkg.jl#3637
   bug precompile +2

[IanButterworth]

Once the code is moved over to Base it's going to be pretty difficult to maintain a version in Pkg on 1.10 which looks likely to become an LTS and in Base on 1.11+, so I've added all the filed issues that I think should be ideally dealt with before moving it over.

The list doesn't have to be completely done, it just makes it easier to do the more are.

[IanButterworth]

One thing to bear in mind with this is that Pkg.precompile does extra Pkg things before precompiling if necessary, like resolve and instantiate. Once the parallel precompilation mechanic is in Base, if we wanted to avoid using Pkg in the code load precompilation path, there would need to be graceful errors if the environment isn't resolved or instantiated, telling the user to use Pkg to do that. That might be a bit of a UX regression.

[KristofferC]

This is done.