go/types, types2: add a test for invalid import of "init"

This error reporting code path did not have test coverage, and panics in
1.17 (filed as #48082).

Add a test that would have reproduced the panic, for both go/types and
cmd/compile/internal/types2.

Change-Id: Icd5f54f8407e4ab57d432f44a129ecf6b2755feb
Reviewed-on: https://go-review.googlesource.com/c/go/+/346309
Trust: Robert Findley <[email protected]>
Run-TryBot: Robert Findley <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: findleyr

src/cmd/compile/internal/types2/testdata/fixedbugs/issue48082.src

@@ -0,0 +1,7 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue48082
+
+import "init" /* ERROR init must be a func */ /* ERROR could not import init */

src/go/types/testdata/fixedbugs/issue48082.src

@@ -0,0 +1,7 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue48082
+
+import "init" /* ERROR init must be a func */ /* ERROR could not import init */