cmd/go/internal/modindex: fix obvious bug using failed type assertion

adonovan pointed out this bug in the review of CL 733320 and this seems
to be the cause of all those list_empty_importpath flakes. It makes
sense that something nondeterministic would be tied up with the module
index because the state of the cache could depend on the order tests are
run in.

Also remove the parts of the test that accept the flaky behavior so we
can verify the issue has been resolved.

For #73976

Change-Id: Ib64ce6b8eed1dc8d327b7c5e842c1e716a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/733321
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Michael Matloob <matloob@golang.org>
Reviewed-by: Michael Matloob <matloob@google.com>

Author: matloob

src/cmd/go/internal/modindex/scan.go

@@ -112,10 +112,10 @@ func parseErrorToString(err error) string {
 		return ""
 	}
 	var p parseError
-	if e, ok := err.(scanner.ErrorList); ok {
-		p.ErrorList = &e
+	if errlist, ok := err.(scanner.ErrorList); ok {
+		p.ErrorList = &errlist
 	} else {
-		p.ErrorString = e.Error()
+		p.ErrorString = err.Error()
 	}
 	s, err := json.Marshal(p)
 	if err != nil {

src/cmd/go/testdata/script/list_empty_importpath.txt

@@ -1,15 +1,6 @@
 ! go list all
 ! stderr 'panic'
-[!GOOS:windows] [!GOOS:solaris] [!GOOS:freebsd] [!GOOS:openbsd] [!GOOS:netbsd] stderr 'invalid import path'
-# #73976: Allow 'no errors' on Windows, Solaris, and BSD until issue
-# is resolved to prevent flakes. 'no errors' is printed by
-# empty scanner.ErrorList errors so that's probably where the
-# message is coming from, though we don't know how.
-[GOOS:windows] stderr 'invalid import path|no errors'
-[GOOS:solaris] stderr 'invalid import path|no errors'
-[GOOS:freebsd] stderr 'invalid import path|no errors'
-[GOOS:openbsd] stderr 'invalid import path|no errors'
-[GOOS:netbsd] stderr 'invalid import path|no errors'
+stderr 'invalid import path'
 
 # go list produces a package for 'p' but not for ''
 go list -e all