Regression test for 'references' bug golang/go#58506.

The 'references' query below, applied to method A.F, implicitly uses
the 'implementation' operation. The correct response includes two
references to B.F, one from package b and one from package d.
However, the incremental 'implementation' algorithm had a bug that
cause it to fail to report the reference from package b.

The reason was that the incremental implementation uses different
algorithms for the local and global cases (with disjoint results), and
that when it discovered that type A satisfies interface B and thus
that B.F must be included among the global search targets, the
implementation forgot to also search package b for local references
to B.F.

-- go.mod --
module example.com
go 1.12

-- a/a.go --
package a

type A int

func (A) F() {} //@loc(refa, "F"), refs("F", refa, refb, refd)

-- b/b.go --
package b

import (
	"example.com/a"
	"example.com/c"
)

type B interface{ F() }

var _ B = a.A(0)
var _ B = c.C(0)

var _ = B.F //@loc(refb, "F")

-- c/c.go --
package c

type C int

// Even though C.F is "rename coupled" to A.F by B.F,
// it should not be among the results.
func (C) F() {}

-- d/d.go --
package d

import "example.com/b"

var _ interface{} = b.B.F //@loc(refd, "F")