x/tools/gopls: plan for vendoring in multi-module workspaces

[stamblerre]

As previously discussed, we have not yet accounted for modules that use vendoring in the workspace module design. As of https://golang.org/cl/259623, the workspace module will be enabled for modules that have vendor directories, but the vendor directories will be ignored.

@heschik investigated the possibility of creating a workspace module vendor directory by combining the vendor directories of all of the modules in the workspace, but this doesn't seem to be a feasible approach.

If we choose to simply ignore vendor directories, we must first resolve #41819. We might also consider using the default mode if we only detect one module in the workspace, as the workspace module will offer no advantage in that case.

[stamblerre]

This is now subsumed by the go.work proposal: #45713.