Implement v2-fetch (project support)

[fgaz]

Describe the feature request

A v2-fetch command that supports projects like the other commands (most recently #10840)

Additional context

https://github.com/haskell/cabal/issues/2242 https://github.com/haskell/cabal/issues/10942 #10935

[ulysses4ever]

~~Dup of https://github.com/haskell/cabal/issues/8584?~~

[fgaz]

It's actually a different command. It can fetch an entire dependency tree

[ulysses4ever]

Oh, my bad!

[mpickering]

@fgaz Just wanted to check you didn't start working on this? Otherwise my colleague has availability to handle it.

[fgaz]

@mpickering Nice! I didn't start working on this. I'm focusing on #6186