[WIP/RFC] add Pkg3 to stdlib

[vchuravy]

I am putting this up as an alternative to #25115 and to garner feedback on this approach even if we decide to do a full sourcetree import of Pkg3.

The idea is to treat external stdlib packages similar to external dependencies, this currently uses git to do the version resolution, but we should probably use tarballs + SHA, with the option to use git for development (similar to how we treat LLVM).

I am by no means a master of Makefiles so feedback on the structure and design would be especially welcomed.

##TODO:

• Make Pkg3 work on current master
• Decide if Pkg3 should be renamed to Pkg
• make Pkg3 work with _Pkg stub
• update docs
• add announcement
• Decision: Should we add dependencies like TOML, TerminalMenues, and BinaryProvider to stdlib

[vchuravy]

I updated the PR by porting some of the changes from #25115 and adding a TODO.

Personally this attempt makes even more sense to me after I realised that Pkg ships copies of TOML, TerminalMenues and BinaryProvider. IMO these packages should be part of the stdlib.

[c42f]

This looks reasonable to me. It's annoying to be in yet another bootstrap situation where the tooling (Pkg3 in this case) can't be used to build itself.

What would presumably be easier would be for an already built version of Pkg3 to emit a consistent set of requirements for the stdlib which could then be hard coded in this repo in a format which this makefile can consume.

Then updating this will be less manual as the stdlib grows to include more modules from various repositories.

[c42f]

(caveat: I know next to nothing about Pkg3!)

[StefanKarpinski]

As soon as #25455 is merged, we can revisit this – we do need to move Pkg3 into Base. The nice thing is that it's really only necessary for generating and manipulating project and manifest files and installing packages into the right place in depot paths, so it can definitely be loaded on-demand.