go.tools/ssa: fix crash in SSA builder when using GCImporter to satis…

…fy imports (ssadump -build=G).

Packages were not being created for all types.Packages,
specifically, indirectly imported packages were missing.
(*Program).CreatePackages now iterates over the type-checker's
package map too.

Also: removed all concurrency from importer.  I think it was
incorrect (and hard to fix).

Also: change LoadInitialPackages so that all named packages
are loaded from source.  This happens regardless of whether
GCImporter is used to satisfy imports.

Details:
- importer.Config.SourceImports flag determines whether to
  load all packages from *.go source.
  (Before, this was indicated by Config.Build != nil.)
- importer.Config.Build field effectively defaults to
  &go/build.Default.  A zero importer.Config is now usable.
- importer.Importer.Config field is now exported.
- LoadPackage renamed to ImportPackage since the resulting
  packages may come from GCImporter (and be incomplete).
- doImport and ImportPackage fused.

Fixes golang/go#7028

R=gri, axwalk
CC=golang-codereviews
https://golang.org/cl/48770043

cmd/ssadump/main.go

@@ -73,7 +73,10 @@ func main() {
 	flag.Parse()
 	args := flag.Args()
 
-	impctx := importer.Config{Build: &build.Default}
+	impctx := importer.Config{
+		Build:         &build.Default,
+		SourceImports: true,
+	}
 	// TODO(adonovan): make go/types choose its default Sizes from
 	// build.Default or a specified *build.Context.
 	var wordSize int64 = 8
@@ -103,7 +106,7 @@ func main() {
 		case 'N':
 			mode |= ssa.NaiveForm
 		case 'G':
-			impctx.Build = nil
+			impctx.SourceImports = false
 		case 'L':
 			mode |= ssa.BuildSerially
 		default:
@@ -147,8 +150,8 @@ func main() {
 
 	// The interpreter needs the runtime package.
 	if *runFlag {
-		if _, err := imp.LoadPackage("runtime"); err != nil {
-			log.Fatalf("LoadPackage(runtime) failed: %s", err)
+		if _, err := imp.ImportPackage("runtime"); err != nil {
+			log.Fatalf("ImportPackage(runtime) failed: %s", err)
 		}
 	}