x/tools/gopls: inform user about excluded/orphaned files due to nested or multiple modules

[stamblerre]

When a workspace contains multiple modules without experimentalWorkspaceModule, some of the modules may be excluded or ignored. We should show a diagnostic on every opened "excluded" file (analogous to the build tag diagnostics) that informs the user of the issue and redirects them to clear documentation on how to add workspace folders or enable experimentalWorkspaceModule.

[gopherbot]

Change https://golang.org/cl/272346 mentions this issue: internal/lsp: improve errors in multi-module workspaces (GO111MODULE=on)

[gopherbot]

Change https://golang.org/cl/275553 mentions this issue: internal/lsp: show multiple modules error message with GO111MODULE=auto

[gopherbot]

Change https://golang.org/cl/275554 mentions this issue: internal/lsp: add an orphaned file diagnostic for nested modules