Tests are failing with go1.19 and newer

[vearutop]

There might a behavior regression in Go, since go1.16 can run the tests successfully, but go1.19 and gotip fail with panic: lost receiver name.

go1.16.10 test ./...
ok      rsc.io/rf       28.153s
ok      rsc.io/rf/diff  0.167s
?       rsc.io/rf/git-generate  [no test files]
ok      rsc.io/rf/refactor      0.168s

go version
go version go1.19.3 darwin/amd64

go test ./...

panic executing: mv x.go f.go
--- FAIL: TestRun (8.76s)
    --- FAIL: TestRun/mv_file4.txt (0.70s)
        rf_test.go:85: testdata/mv_file4.txt
panic: lost receiver name [recovered]
        panic: lost receiver name [recovered]
        panic: lost receiver name

goroutine 434 [running]:
testing.tRunner.func1.2({0x12a71c0, 0x135e9d0})
        /usr/local/opt/go/libexec/src/testing/testing.go:1396 +0x24e
testing.tRunner.func1()
        /usr/local/opt/go/libexec/src/testing/testing.go:1399 +0x39f
panic({0x12a71c0, 0x135e9d0})
        /usr/local/opt/go/libexec/src/runtime/panic.go:884 +0x212
rsc.io/rf.run.func1()
        /Users/vearutop/dev/rf/rf.go:76 +0xa5
panic({0x12a71c0, 0x135e9d0})
        /usr/local/opt/go/libexec/src/runtime/panic.go:884 +0x212
rsc.io/rf/refactor.(*Snapshot).addPkgDeps(0x0?, 0xc0008db909?, 0xc009f09ad0)
        /Users/vearutop/dev/rf/refactor/pkgref.go:223 +0x46f
rsc.io/rf/refactor.(*Snapshot).DepsGraph(0xc00012aab0, 0x2)
        /Users/vearutop/dev/rf/refactor/pkgref.go:147 +0x6d
rsc.io/rf.mvCode(0xc00012aab0, {0xc00c8a6bb0, 0x1, 0xc00ad91da0?}, 0xc000058110?, 0xc00ae8aea0)
        /Users/vearutop/dev/rf/mvcode.go:122 +0x59d
rsc.io/rf.cmdMv(0xc00012aab0, {0xc004838303?, 0x12f9946?})
        /Users/vearutop/dev/rf/mv.go:127 +0xb7a
rsc.io/rf.run(0xc0031f27e0, {0xc0048382a0?, 0xd?})
        /Users/vearutop/dev/rf/rf.go:133 +0x2d1
rsc.io/rf.TestRun.func1(0xc008f82b60)
        /Users/vearutop/dev/rf/rf_test.go:121 +0x345
testing.tRunner(0xc008f82b60, 0xc00beae6a0)
        /usr/local/opt/go/libexec/src/testing/testing.go:1446 +0x10b
created by testing.(*T).Run
        /usr/local/opt/go/libexec/src/testing/testing.go:1493 +0x35f
FAIL    rsc.io/rf       8.927s
ok      rsc.io/rf/diff  (cached)
?       rsc.io/rf/git-generate  [no test files]
ok      rsc.io/rf/refactor      (cached)
FAIL