feat: align with upcoming virtual package cep

[baszalmstra]

This PR aligns our virtual package implementation with the upcoming virtual package cep. I think most of the changes proposed in the CEP make a lot of sense so I feel we can already merge this PR before the CEP is accepted.

In particular, I made the following changes:

• __win now also reports the version of Windows.
• __win can be overridden with CONDA_OVERRIDE_WIN
• __linux can be overridden with CONDA_OVERRIDE_LINUX
• __archspec can be overriden with CONDA_OVERRIDE_ARCHSPEC
• __archspec falls back to deriving the archspec from the platform if it could not be determined.
• __archspec is added as __archspec=1=0 if the no archspec could be determined for the current platform.

Changes I explicitly did not make (also left a question in the CEP):

• osx-arm64 should fall back to arm64 archspec but we use m1 instead.
• win-arm64 should fall back to arm64 archspec but we use aarch64 instead.

I left this in because arm64 is not an existing archspec and m1 and aarch64 are the base microarchitectures for osx-arm64 and win-arm64 respectively. I think this will yield better defaults.

I also added a VirtualPackages struct that contains all the virtual package implementations and provides handy utilities to convert it to a list of VirtualPackage or a list of GenericVirtualPackage. Detection of virtual packages is now also handled by this struct.

Tagging @jaimergp so he is aware of this!