Add support for monorepos with versions set to "workspace:*"

[gr2m]

Hi there 👋🏼

Love the project! I'm working on https://github.com/vercel/ai and would love to enable users to install packages directly from pull requests for testing. It's a monorepo and dependencies from other workspaces are referenced using "workspace:*".

Sicn gitpkg can already alter package.json files, I wonder if you'd be open to implement a logic which would replace these placeholders with the latest stable version of the respective package? Or alternatively allow to override a version value for a dependency via a parameter?