x/tools/gopls: completing inside `new()` suggests keywords first

[muirdm]

package main

func main() {
	var _ *int = new()
}

Completing inside new() should offer int first, but it instead offers chan, func, map, and then int.

Probably addKeywordCompletions shouldn't add keyword type name candidates if there is already an expected type (int is the expected type in the above example).

[gopherbot]

Change https://golang.org/cl/246360 mentions this issue: internal/lsp/source: improve unnamed type completion