go/ssa: reduce set of expected indirect imports

...now that gc's export data records fewer dependencies.

Change-Id: Ie0f68f1f7b0825f6d2c100dfe189be017383db60
Reviewed-on: https://go-review.googlesource.com/22584
Reviewed-by: Robert Griesemer <gri@golang.org>

go/ssa/builder_test.go

@@ -69,8 +69,9 @@ func main() {
 	deps := []string{
 		// directly imported dependencies:
 		"bytes", "io", "testing",
-		// indirect dependencies (partial list):
-		"errors", "fmt", "os", "runtime",
+		// indirect dependencies mentioned by
+		// the direct imports' export data
+		"sync", "unicode", "time",
 	}
 
 	prog := mainPkg.Prog