fix: Resolve generator name conflicts across workspaces (#12467)
## Summary
Fixes #6215
- When multiple workspaces define a generator with the same name (e.g.,
`component`), only one would show up in `turbo gen` because all configs
were loaded into a single `node-plop` instance — the last one silently
overwrote earlier registrations.
- Each workspace config now gets its own plop instance during discovery,
so same-named generators from different workspaces coexist correctly.
- Generators carry a qualified identifier (`workspace/name`) that's
displayed in the selection prompt and used for unambiguous lookup.
Unqualified names still work when there's only one match (backward
compat).
## How to test
1. Create two workspaces with `turbo/generators/config.ts`, both
registering a generator named `component`
2. Run `turbo gen`
3. **Before**: only one `component` generator visible
4. **After**: both visible as `ui-kit/component` and `webapp/component`
under their workspace headers
Passing a generator name via CLI (`turbo gen run component`) still works
when unambiguous. When ambiguous, you get a helpful message listing the
qualified alternatives.
