x/tools/gopls: use overlays when computing mod tidy diagnostics

[findleyr]

We currently invalidate mod tidy results whenever metadata changes, but compute diagnostics based on saved files. This can be very confusing as go.mod diagnostics do not match the current workspace state.

With native overlay support, we should be able to make mod tidy diagnostics honor unsaved changes, and should explore updating mod tidy diagnostics whenever metadata changes.