x/tools/gopls: don't recreate the view when a go.mod file is created

[stamblerre]

This caused golang/vscode-go#363, because a go.mod was being created during that test, and the completion request was reaching the server at the time that the view was shutting down.

We either need to handle such a situation more gracefully, or we shouldn't shut down a view on go.mod creation. That would mean putting a mutex around a lot of the view's fields that we treat as immutable.

[gopherbot]

Change https://golang.org/cl/245818 mentions this issue: internal/lsp: add a test for golang/go#37069