Allow cfg's in workspaces

[Kixiron]

Describe the problem you are trying to solve
Cargo should allow wider support for cfgs in the Cargo.toml
For example, I have a workspace member b which I only want to compile for the windows platform, and my intuition tells me that this should work

[target.'cfg(windows)'.workspace]
exclude = ["crates/b"]

However, cargo ignores the cfg in this position, forcing me to manually comment out the exclude in order to compile for other platforms

[epage]

#6179 has solving this as one of its goals. I'm closing in favor of that to consolidate the discussion and as its the more likely route for us to go. If there is a reason for us to reconsider this, let us know!