cmd/go: fix import directory list for compilation

This fixes the most annoying bug in the go command,
that 'go build' sometimes ignored packages it had just
rebuilt in favor of stale installed ones.

This part of the code needs more thought, but this small
change is an important improvement.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/5531053

src/cmd/go/build.go

@@ -509,9 +509,9 @@ func (b *builder) build(a *action) error {
 	incMap[build.Path[0].PkgDir()] = true // goroot
 	incMap[""] = true                     // ignore empty strings
 
-	// build package directories of dependencies
+	// temporary build package directories of dependencies.
 	for _, a1 := range a.deps {
-		if pkgdir := a1.pkgdir; !incMap[pkgdir] {
+		if pkgdir := a1.pkgdir; pkgdir != a1.p.t.PkgDir() && !incMap[pkgdir] {
 			incMap[pkgdir] = true
 			inc = append(inc, "-I", pkgdir)
 		}
@@ -522,7 +522,7 @@ func (b *builder) build(a *action) error {
 
 	// then installed package directories of dependencies
 	for _, a1 := range a.deps {
-		if pkgdir := a1.p.t.PkgDir(); !incMap[pkgdir] {
+		if pkgdir := a1.p.t.PkgDir(); pkgdir == a1.pkgdir && !incMap[pkgdir] {
 			incMap[pkgdir] = true
 			inc = append(inc, "-I", pkgdir)
 		}