x/tools/gopls: rename: spurious pkgname conflict in xtest

[adonovan]

The renamer reports a conflict where there would be none, when renaming AA in the program below to aa:

xtools$ pwd
/Users/adonovan/w/xtools

xtools$ cat aa/aa.go 
package aa

var AA int // renaming this var "AA" to "aa" would conflict...

const C = 0

xtools$ cat aa/aa_test.go 
package aa_test

import "golang.org/x/tools/aa" // ...with this imported package name

var _ = aa.C

xtools$ gopls rename ./aa/aa.go:#17 aa     # rename AA to aa
gopls: aa/aa.go:3:5: renaming this var "AA" to "aa" would conflict
aa/aa_test.go:3:8:	with this imported package name