Support manage packages for global.

[frostming]

From https://github.com/pdm-project/call-for-features/issues/1

1. Place a global-project folder under PDM app direcctory.
2. Add a command group global that supports following subcommands:
   • install
   • sync
   • remove
   • add
   • update
   • list

Any to add? @nasyxx

[nasyxx]

Well, and the command shows the path of the global-project folder.

[nasyxx]

In fact, I have thought a lot about this again. Should we add a run command?

Suppose we add a global package which installs a script/executable file, should we always use pdm run exe to run it? Or we should only install/link it to $PDM.D/bin/?

This may be another feature comes with the global-project that can be used instead of pipx.

[frostming]

My understanding is global commands are just like you use the global pip or python, adding dependency resolution and tracking. Executables are run like before, assuming you have added the bin path to PATH.

However, pipx is for isolated installation, so that the package won't mix with global space. Although you can achieve the same thing by initialize a project, install dependencies, and link the executable at __packages__/X.Y/bin to PATH, there are still more actions than pipx to do. So I recommend using pipx directly.