go/analysis/passes/ctrlflow: add test of modularity

See the Modularity section of the design doc:
https://docs.google.com/document/d/1-azPLXaLgTCKeKDNg0HVMq2ovMlD-e7n1ZHzZVzOlJk#heading=h.s7mcpao0dpqu

Change-Id: I83e6df01691964703a07a47c222101f3307a0dc6
Reviewed-on: https://go-review.googlesource.com/c/140759
Reviewed-by: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>

go/analysis/passes/ctrlflow/testdata/src/a/a.go

@@ -8,6 +8,8 @@ import (
 	"runtime"
 	"syscall"
 	"testing"
+
+	"lib"
 )
 
 var cond bool
@@ -97,3 +99,11 @@ func spurious() { // want spurious:"noReturn"
 }
 
 func noBody()
+
+func g() {
+	lib.CanReturn()
+}
+
+func h() { // want h:"noReturn"
+	lib.NoReturn()
+}

go/analysis/passes/ctrlflow/testdata/src/lib/lib.go

@@ -0,0 +1,8 @@
+package lib
+
+func CanReturn() {}
+
+func NoReturn() {
+	for {
+	}
+}