x/tools/gopls: verify support for workspace vendoring

[findleyr]

https://go.dev/issue/60056 is adding support for workspace vendoring.

This is likely to work out-of-the-box since gopls relies on go list for package information. However, we'll need to verify that this works well and include additional regression tests for the new workspace layout.

We may want to add code lenses to update the vendor directory.

[gopherbot]

Change https://go.dev/cl/560466 mentions this issue: gopls/internal/cache: support workspace vendoring

[gopherbot]

Change https://go.dev/cl/560717 mentions this issue: gopls/internal/cache: detect and reinit on workspace vendor changes