go/build: reject directory with only cgo files if cgo not in use

The old test for "no Go files" was p.Name == "", meaning we never
saw a Go package statement. That test fails if there are cgo files
that we parsed (and recorded the package name) but then chose
not to use (because cgo is not available).

Test the actual file lists instead.

Fixes #6078.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/13661043

src/cmd/go/test.bash

@@ -167,6 +167,19 @@ elif ! grep testdata/shadow/root1/src/foo testdata/err >/dev/null; then
 fi
 unset GOPATH
 
+TEST go install fails with no buildable files
+export GOPATH=$(pwd)/testdata
+export CGO_ENABLED=0
+if ./testgo install cgotest 2>testdata/err; then
+	echo "go install cgotest succeeded unexpectedly"
+elif ! grep 'no buildable Go source files' testdata/err >/dev/null; then
+	echo "go install cgotest did not report 'no buildable Go source files'"
+	cat testdata/err
+	ok=false
+fi
+unset CGO_ENABLED
+unset GOPATH
+
 # Test that without $GOBIN set, binaries get installed
 # into the GOPATH bin directory.
 TEST install into GOPATH

src/cmd/go/testdata/src/cgotest/m.go

@@ -0,0 +1,5 @@
+package cgotest
+
+import "C"
+
+var _ C.int

src/pkg/go/build/build.go

@@ -747,6 +747,8 @@ Found:
 			allTags["cgo"] = true
 			if ctxt.CgoEnabled {
 				p.CgoFiles = append(p.CgoFiles, name)
+			} else {
+				p.IgnoredGoFiles = append(p.IgnoredGoFiles, name)
 			}
 		} else if isXTest {
 			p.XTestGoFiles = append(p.XTestGoFiles, name)
@@ -756,7 +758,7 @@ Found:
 			p.GoFiles = append(p.GoFiles, name)
 		}
 	}
-	if p.Name == "" {
+	if len(p.GoFiles)+len(p.CgoFiles)+len(p.TestGoFiles)+len(p.XTestGoFiles) == 0 {
 		return p, &NoGoError{p.Dir}
 	}