[Feature]: `yarn workspaces focus` should make all subsequent operations "focused"

• I'd be willing to implement this feature
• This feature can already be implemented through a plugin

Describe the user story

Essentially https://twitter.com/VLecrubier/status/1301620546589913088.

Our current workflow when building a module/artifact on CI (with yarn v1, fwiw) is:

• a custom script that does yarn workspaces info and finds a package and its dependencies - it then runs rm -rf on all other packages and runs yarn in root (so essentially yarn workspaces focus, but will modify lockfile)
• we then use oao to run yarn build in all packages which has that command following the inverse dependency tree etc.

That last step is still needed if we were to migrate to Yarn v2 as we cannot do yarn workspaces foreach -pt run build as yarn will run that in all workspaces rather than the ones focus has... focused.

Describe the solution you'd like

After running yarn workspace focus all further yarn workspaces * commands should respect the fact we're currently "focused" and not do work on all workspaces. I'm specifically after yarn workspaces foreach run, but I think other commands like yarn why or yarn up should also only operate on the "focused" workspace and its dependencies.

As an aside, for this I think it'd make more sense if focus just "focused" without installing anything. So yarn workspaces focus foobar would just set some somewhere in .yarn/ I guess, and when we then run yarn install afterwards, only foobar (and its dependencies) would be "installed".

Describe the drawbacks of your solution

Not sure... It would probably mean more forking of logic ("all workspaces" vs "some workspaces") in multiple places, but I don't think there's any drawbacks beyond the implementation aspect of it?

Describe alternatives you've considered

Not use yarn workspaces focus at all as it doesn't work for our use cases and keep using a manual approach.

Additional context

Happy to chat on Discord about what other stuff we do on CI to build if applicable.

Start by tracing the command entry points for yarn workspaces focus and yarn workspaces foreach run, then inspect how workspace selection is represented. Done means subsequent workspace operations consistently honor the focused workspace and its dependencies, including the requested foreach behavior.