x/tools/gopls: improve UX with unimported completions and -mod=readonly

[stamblerre]

The new -mod=readonly changes have affected the behavior of unimported completions, leading to some strange behavior. Here's an example of the problem:

1. Create a new module. github.com/yuin/goldmark is already in your local module cache, but not in the go.mod file of the module.
2. Create a main.go file and start typing goldmark. You will see a completion for goldmark suggested.

3. Accept the completion and add a . to trigger a subsequent completion.

4. See no results because the new package has not yet been added to the go.mod file. However, no diagnostic is shown on the import because the file is still unsaved.

5. Typing any characters, even if they are incorrect, will cause the diagnostic to show up.

Also, note that goimports works fine without the require in the go.mod file, but unimported completions fail in the specific case of an accepted unimported package completion followed by a . -- saving the file as-is causes the diagnostic to appear, but it does not cause unimported completions to appear.

/cc @findleyr @heschik

[myitcv]

This is roughly the same scenario I mentioned when discussing automatically accepting changes to go.mod in the context of govim. Because my argument is that accepting of the completion in step 3 is tantamount to automatically accepting that the requirement should be added to your go.mod (with an implied local @latest, because it's not guaranteed to be the real @latest)

[stamblerre]

IMO, the fixes needed here are to (1) show the diagnostic on the unsaved file (and figure out why it's not showing up), and (2) to make sure that unimported completions work even if the module has not yet been added to the go.mod file.

[gopherbot]

Change https://golang.org/cl/280095 mentions this issue: internal/lsp: fix unimported completions with -mod=readonly

[stamblerre]

I'm not able to reproduce this simply in the playground so this may take some more investigation: https://play.golang.org/p/7NupO_XLpKX.

[stamblerre]

My mistake--it actually is reproducible with a simple example: https://play.golang.org/p/j5y5HtJ8Afm. Looks like this is WAI.