x/tools/gopls: reuse named imports from other packages in the current module

[mvdan]

Take the following dummy module with two simple Go files:

-- go.mod --
module test

go 1.14
-- foo/foo.go --
package main

import strs "strings"

var _ = strs.Count
-- bar/bar.go --
package main

var _ = strs.Builder

If I open both Go files with vim+govim+gopls and save, I would expect bar.go to gain import strs "strings", mimicking the other package in the same module. This works fine if both files are in the same package, but not if they are in different packages.

Assuming that gopls already collects information about all packages in the current main module, I think it should also be able to pick up named imports from other packages to try to be more helpful when adding imports.

In particular, this came up while writing tests in a new package with import qt "github.com/frankban/quicktest".

cc @heschik @stamblerre @myitcv @danp