x/tools/gopls: variadic completion ranking incorrect with interfaces

[stamblerre]

Simplified repro:

var cphs []source.CheckPackageHandle
var handles []*checkPackageHandle
for _, x := range handles {
    cphs = append(cphs, <>)
}

I got a suggestion for cphs... above x. This is probably because *checkPackageHandle only implements source.CheckPackageHandle.

/cc @muirdm

[gopherbot]

Change https://golang.org/cl/207518 mentions this issue: internal/lsp: fix variadic interface completion ranking