x/tools/gopls: channels are not ranked highest in parameters to close

[stamblerre]

Repro:

package main

var xb chan int

func xa() int { return 0 }

func main() {
	close(<>)
}

xa will be ranked above xb.

/cc @muirdm

[gopherbot]

Change https://golang.org/cl/212957 mentions this issue: internal/lsp/source: improve completion support for args to builtins