Build dependencies only

[deemp]

Related to #1180.

As spago supports offline builds, it's now possible to use spago in Nix derivations.

If a package has some non-local and local dependencies, I'd like to build them and put into a derivation and then use that derivation for building the package derivation. This way, I'll be able to avoid rebuilding dependencies when the package code changes.

However, for spago 0.93.24, I only see:

• spago install that builds non-local dependencies
• spago build that builds the package and its dependencies

So, I can't build only local dependencies in a derivation. I have to run spago build for each local dependency, which isn't particularly convenient.

Moreover, I have to include the package directory into dependencies derivation. Hence, when I change a file in that directory, the whole dependencies derivation has to be rebuilt.

[bakhtiyarneyman]

+1.

IMHO it would be fantastic if spago facilitated cargo2nix-style (1 nix package per 1 spago package or workspace) rather than crane-style (1 package with all of deps) building.