Use new API to resolve workspaces server-side

Created by: eseliger

Makes use of the newly introduced endpoint in src-cli. This is usually faster, and it means we can eventually have just one code path again.

I think this is good as-is and a net-improvement that we should get in, but before we can remove the old code path, we should have a conversation about the src batch repos command:

• We cannot render the same output with the new resolver
• Because the current output is incorrect. It doesn't consider workspaces, nor ignored repos or any potential overwrites by a later rule in the on: section Hence I propose that we shift this to run a proper workspace resolution and just print that outcome, like we do in the UI.

Closes https://github.com/sourcegraph/sourcegraph/issues/36261

Test plan

Validated everything works as expected.