Revert "cmd/build: reject non-existant directories in ImportDir"

This reverts commit 7af2ce3f.

The commit had a wrong prefix in the description line, probably
misreconginized something. As a result it broke golang.org/x/tools/godoc
and golang.org/x/tools/cmd/godoc like the following:

--- FAIL: TestCLI (10.90s)
--- FAIL: TestWeb (13.74s)
FAIL
FAIL        golang.org/x/tools/cmd/godoc    36.428s
--- FAIL: TestCommandLine (0.00s)
FAIL
FAIL        golang.org/x/tools/godoc        0.068s

Change-Id: I362a862a4ded8592dec7488a28e7a256adee148f
Reviewed-on: https://go-review.googlesource.com/23076
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
Reviewed-by: Andrew Gerrand <adg@golang.org>

src/go/build/build.go

@@ -403,11 +403,7 @@ func (p *Package) IsCommand() bool {
 // ImportDir is like Import but processes the Go package found in
 // the named directory.
 func (ctxt *Context) ImportDir(dir string, mode ImportMode) (*Package, error) {
-	p, err := ctxt.Import(".", dir, mode)
-	if err == nil && !ctxt.isDir(p.Dir) {
-		err = fmt.Errorf("%q is not a directory", p.Dir)
-	}
-	return p, err
+	return ctxt.Import(".", dir, mode)
 }
 
 // NoGoError is the error used by Import to describe a directory

src/go/build/build_test.go

@@ -5,7 +5,6 @@
 package build
 
 import (
-	"fmt"
 	"internal/testenv"
 	"io"
 	"os"
@@ -346,13 +345,3 @@ func TestImportVendorParentFailure(t *testing.T) {
 		t.Fatalf("error on failed import does not mention GOROOT/src/vendor directory:\n%s", e)
 	}
 }
-
-// Issue 3248
-func TestBogusDirectory(t *testing.T) {
-	const dir = "/foo/bar/baz/gopher"
-	_, err := ImportDir(dir, FindOnly)
-	want := fmt.Sprintf("%q is not a directory", filepath.FromSlash(dir))
-	if err == nil || err.Error() != want {
-		t.Errorf("got error %q, want %q", err, want)
-	}
-}