feat: generic workspace support

[mistydemeo]

This adds workspace support to generic builds. Prior to this, we only considered the root dist.toml and had no support for multi-project workspaces.

This doesn't yet have support for mixed workspaces where packages of different languages are part of the same workspace, but it is possible for repos of different languages to be contained within the same repo so long as there's both a dist.toml with a workspace for generic projects and a Cargo.toml with a workspace for Cargo projects. For example:

searching for generic project...
manifest: /private/tmp/axolotlsay-hybrid/dist.toml
target: /private/tmp/axolotlsay-hybrid/target
  axolotlsay-bun@0.3.0
    manifest: /private/tmp/axolotlsay-hybrid/axolotlsay-bun/dist.toml
    [bin] axolotlsay-bun
  axolotlsay-js@0.3.0
    manifest: /private/tmp/axolotlsay-hybrid/axolotlsay-js/dist.toml
    [bin] axolotlsay-js

searching for rust project...
manifest: /private/tmp/axolotlsay-hybrid/Cargo.toml
target: /private/tmp/axolotlsay-hybrid/target
  axolotlsay@0.2.1
    manifest: /private/tmp/axolotlsay-hybrid/axolotlsay/Cargo.toml
    [bin] axolotlsay

This contains a breaking API change - I previously incorrectly put the build_command in the workspace definition for generic projects, which was wrong. It's been moved out of the workspace info and into the package info. This will require minor code changes in cargo-dist to use.