Handle an additional vendor directory

Additional gb.cfg config
pkgdir=vendor

When building the workspace $pkgdir will removed from
the packages target

W/$pkgdir/a/b/c -> a/b/c

gb/config.go

@@ -79,6 +79,11 @@ func (cfg Config) GCFlags() (gcflags string, set bool) {
 	return
 }
 
+func (cfg Config) Pkgdir() (pkgdir string, set bool) {
+	pkgdir, set = cfg["pkgdir"]
+	return
+}
+
 func (cfg Config) Write(dir string) (err error) {
 	path := filepath.Join(dir, "gb.cfg")
 	var fout *os.File
@@ -118,6 +123,7 @@ var knownKeys = map[string]bool{
 	"ignore":    true,
 	"ignoreall": true,
 	"gcflags":   true,
+	"pkgdir":    true,
 }
 
 func ReadConfig(dir string) (cfg Config) {

gb/pkg.go

@@ -584,7 +584,16 @@ func (this *Package) GetTarget() (err error) {
 					}
 					return false
 				}
-				_ = tryFixPrefix(path.Join("src", "pkg")) || tryFixPrefix("pkg") || tryFixPrefix("src")
+				fixed := tryFixPrefix(path.Join("src", "pkg")) || tryFixPrefix("pkg") || tryFixPrefix("src")
+
+				for t := this; t != nil; t = t.Parent {
+					if pkgdir, set := t.Cfg.Pkgdir(); set {
+						if !fixed {
+							tryFixPrefix(pkgdir)
+						}
+						break
+					}
+				}
 			}
 		} else {
 			this.Base = this.Target