Resolve feature and optional dependencies for workspace as a whole

[ishitatsuyuki]

Currently, a dependency's feature set is determined by what package is getting built, without considering other packages in the workspace. This will likely cause a crate to be compiled more than once with different feature sets.

Instead, we should consider the full workspace when resolving features. We have a few choices:

• Assuming all of the features are enabled when resolving/compiling dependencies
  This will minimize the possibility we will be building a crate twice, but this can be unwanted behavior sometimes.
• Assuming default features for all crate in the workspace
  This will be less surprising though it doesn't cover all the feature combinations that are possible.

Note that specifying features in workspace doesn't work anyway ¯\_(ツ)_/¯: #5015

Related: #4463

[epage]

As a workaround, there is https://crates.io/crates/cargo-hakari to help create a "workspace hack" that enables all of the relevant features for all packages.

We have competing needs

• Workspaces that publish their content independently need to be able to have their features tested individually
• Workspaces with a product broken into packages need a quick way to move around the product and build pieces without rebuilding dependencies with different features selected

The challenge will be in having a solution that easily let's people navigate between those needs, including when they co-exist within the same workspace.