x/tools/gopls: autocomplete in import blocks

[stamblerre]

Suggested in microsoft/vscode-go#2918.

[muirdm]

Is there a use case for this once unimported completions are enabled by default? The only case I can think of is manually correcting an (incorrect) ambiguous import, but even that should be pretty rare.

[heschik]

I agree the justification for this is weak. Maybe if you're exploring an import hierarchy, like you can't remember the exact package name you're looking for but you know what repo it was in?

[stamblerre]

@heschik: That's what I was thinking. Definitely not pressing, but I thought it wasn't a bad idea.