x/tools/gopls: revisit the inconsistent vendoring error diagnostics message

[hyangah]

I was originally about to propose to move the issue URL to LSP Diagnostics CodeDescription href field (due to the limitation in vscode UI, links in diagnostics message can be easily clicked or copied otherwise). However, reading the issue details, I am not sure how helpful the linked issue is to general audience. Improvement I think of

• Explain what is happening - e.g. "this error prohibits the language server from function correctly. Please fix this issue by running "go mod vendor".
• Have a better information page that explains what is inconsistent vendoring means, and what's the impact on the editor's functionality (and from there, link the issue 39164 if we want) and supply the link using CodeDescription href field.