[Feature] yarn why for individual workspace

[BasixKOR]

• I'd be willing to implement this feature (contributing guide)
• This feature is important to have in this repository; a contrib plugin wouldn't do

Describe the user story

We have a huge monorepo consisting of a lot of workspaces, and yarn why result is often too long to be used quickly & easily, especially if the multiple version of same package is used throughout the project.

Describe the solution you'd like

• Option A: Change yarn why behavior to only look for current workspace's recursive dependencies.
  • Breaking change!
• Option B: Add a flag to opt into the behavior specified in A.

Describe the drawbacks of your solution

I don't see a particular drawbacks, except option A may cause breaking changes.

Describe alternatives you've considered

Making this into a plugin is certainly possible, but I think looking into specific workspace's dependencies is general enough to be added into Yarn itself.