towards Nupm workspaces

i was once again working on NGM and was thinking about a few things related to Nupm 😏

• installing multiple modules in a single package:
  in https://github.com/nushell/nupm/pull/33, we made it possible to install multiple distinct modules from a single package, e.g. nu-git-manager and nu-git-manager-sugar in NGM. However, after using this feature, it sounds a bit strange... it's a bit like having two separate src/ directories in a single Rust crate 🤔
  i think we should revert that feature and keep it simpler, i.e. a single module per module package, and move towards a "workspace" or "bundle" model
  • https://github.com/nushell/nupm/pull/50
• dependencies resolution:
  in NGM, i always have to write things like use ../../foo/bar.nu in the modules themselves or use ../src/nu-git-manager/ in the tests, which is not that great 🤔
  i think it would be great to have a nupm run that would resolve the dependencies and do that for the user, e.g. use module/foo/bar.nu and use nu-git-manager respectively: this would only be a trick with NU_LIB_DIRS to expose modules and activate overlays
  • https://github.com/nushell/nupm/pull/52
• "workspace" or "bundle" support:
  would be cool to have multiple module packages in a repo and a central top-level package.nuon that would define a "workspace" and allow maybe to install everything at once?, e.g. nupm install --workspace
• rename package.nuon to nupm.nuon
  • https://github.com/nushell/nupm/pull/51