MAINT micropip: move most methods from PackageManager to Transaction and remove PackageManager

[hoodmane]

This is more reorganization of micropip to try to make the logic easier to follow. I turned INSTALLED_PACKAGES into a global variable. I turned _install into a top level function definition and merged it with install rather than a class method. The other methods of PackageManager turned into Transaction methods. I removed PackageManager entirely.

[hoodmane]

I'm not really sure why matplotlib fails to build on this branch...

[ryanking13]

I turned _install into a top level function definition and merged it with install rather than a class method.

I was always thinking of this but never worked on it, thanks @hoodmane!
I didn't review the detail, I guess code details are not changed much right?

There is a failure in CI, but the matplotlib build failure doesn't seem related to this PR.

[hoodmane]

Yeah it seems that matplotlib is failing on a bunch of my branches with micropip-related changes.

code details are not changed much right?

No, it's just a refactoring. I consolidated PackageManager.install with install functions, moved the other methods from PackageManager to Transaction, and moved PackageManager.installed_packages to INSTALLED_PACKAGES global variables.

[hoodmane]

Thanks for the review @ryanking13!

[ryanking13]

It seems like recent release of setuptools is causing an error #2567