cmd/doc: skip directories like other go tools

It was skipping dirs starting with ".", but it was missing the "_"
prefix and the "testdata" name. From "go help packages":

	Directory and file names that begin with "." or "_" are ignored
	by the go tool, as are directories named "testdata".

Before the change:

	$ go doc z # using src/cmd/go/testdata/testvendor/src/q/z
	package z // import "."

After the fix, it falls back to the current directory, as expected when
a single argument isn't found as a package in $GOPATH.

TestMain needs a small adjustment to keep the tests working, as now
their use of cmd/doc/testdata would normally not work.

Fixes #24462.

Change-Id: I1f5d6d1eba0fb59aff55db33b3b1147e300284ef
Reviewed-on: https://go-review.googlesource.com/106935
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/doc/doc_test.go

@@ -16,10 +16,20 @@ import (
 )
 
 func TestMain(m *testing.M) {
-	// otherwise the tests are brittle, as they may give unexpected
-	// output or errors when a suffix match with GOPATH takes place
+	// Clear GOPATH so we don't access the user's own packages in the test.
 	buildCtx.GOPATH = ""
+
 	dirsInit()
+
+	// Add $GOROOT/src/cmd/doc/testdata explicitly so we can access its contents in the test.
+	// Normally testdata directories are ignored, but sending it to dirs.scan directly is
+	// a hack that works around the check.
+	testdataDir, err := filepath.Abs("testdata")
+	if err != nil {
+		panic(err)
+	}
+	go func() { dirs.scan <- testdataDir }()
+
 	os.Exit(m.Run())
 }