Replace build_workspace fn with an initialize function that takes a workspace

This makes it clearer that the function is not providing necessary
dependencies to a workspace, but rather configuring it with all of
the panels and widgets which are defined in downstream crates.

crates/collab/src/rpc.rs

@@ -6598,7 +6598,7 @@ mod tests {
                 themes: ThemeRegistry::new((), cx.font_cache()),
                 fs: FakeFs::new(cx.background()),
                 build_window_options: || Default::default(),
-                build_workspace: |_, _, _| unimplemented!(),
+                initialize_workspace: |_, _, _| unimplemented!(),
             });
 
             Channel::init(&client);