cmd/gc: improve syntax error for "import testing"

for this program:
package A
import testing

old diagnostics:
pkg.go:2: syntax error: unexpected semicolon or newline, expecting string literal

now:
pkg.go:2: syntax error: missing import path; require quoted string

Fixes #5332.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/9393043

src/cmd/gc/go.errors

@@ -17,6 +17,9 @@ static struct {
 	% loadsys package LIMPORT '(' LLITERAL import_package import_there ','
 	"unexpected comma during import block",
 
+	% loadsys package LIMPORT LNAME ';'
+	"missing import path; require quoted string",
+
 	% loadsys package imports LFUNC LNAME '(' ')' '{' LIF if_header ';'
 	"unexpected semicolon or newline before {",
 

src/cmd/gc/yerr.h

@@ -17,6 +17,9 @@ static struct {
 	221, ',',
 	"unexpected comma during import block",
 
+	32, ';',
+	"missing import path; require quoted string",
+
 	377, ';',
 	"unexpected semicolon or newline before {",