What's the problem this PR addresses?

I'm working on a way for projects to add dependencies on git repositories while still resolving workspaces properly. For that to work, yarn pack needs to keep the workspace: prefixes rather than turn them into their semver counterparts.

How did you fix it?

Implements a --preserve-workspaces flag that instructs Yarn to keep the workspaces unchanged.

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.