cmd/doc: search for packages in the two-arg case

When given one argument, as in

	go doc binary.BigEndian

doc would search for the package, but when given two, as in

	go doc binary BigEndian

it would not. Fix the inconsistency.

Fixes #18697
Fixes #18664

Change-Id: Ib59dc483e8d4f91e6061c77a5ec24d0a50e115f0
Reviewed-on: https://go-review.googlesource.com/59413
Reviewed-by: Aliaksandr Valialkin <valyala@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/doc/doc_test.go

@@ -541,6 +541,36 @@ func TestMultiplePackages(t *testing.T) {
 	}
 }
 
+// Test the code to look up packages when given two args. First test case is
+//	go doc binary BigEndian
+// This needs to find encoding/binary.BigEndian, which means
+// finding the package encoding/binary given only "binary".
+// Second case is
+//	go doc rand Float64
+// which again needs to find math/rand and not give up after crypto/rand,
+// which has no such function.
+func TestTwoArgLookup(t *testing.T) {
+	if testing.Short() {
+		t.Skip("scanning file system takes too long")
+	}
+	maybeSkip(t)
+	var b bytes.Buffer // We don't care about the output.
+	{
+		var flagSet flag.FlagSet
+		err := do(&b, &flagSet, []string{"binary", "BigEndian"})
+		if err != nil {
+			t.Errorf("unexpected error %q from binary BigEndian", err)
+		}
+	}
+	{
+		var flagSet flag.FlagSet
+		err := do(&b, &flagSet, []string{"rand", "Float64"})
+		if err != nil {
+			t.Errorf("unexpected error %q from rand Float64", err)
+		}
+	}
+}
+
 type trimTest struct {
 	path   string
 	prefix string