cmd/compile: add -importmap option

The -importmap option takes an argument of the form old=new
and specifies that import "old" should be interpreted as if it said
import "new". The option may be repeated to specify multiple mappings.

This option is here to support the go command's new -vendor flag.

Change-Id: I31b4ed4249b549982a720bf61bb230462b33c59b
Reviewed-on: https://go-review.googlesource.com/10922Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/compile/doc.go

@@ -54,6 +54,9 @@ Flags:
 		Remove the limit on the number of errors reported (default limit is 10).
 	-h
 		Halt with a stack trace at the first error detected.
+	-importmap old=new
+		Interpret import "old" as import "new" during compilation.
+		The option may be repeated to add multiple mappings.
 	-installsuffix suffix
 		Look for packages in $GOROOT/pkg/$GOOS_$GOARCH_suffix
 		instead of $GOROOT/pkg/$GOOS_$GOARCH.

src/cmd/compile/internal/gc/lex.go

@@ -214,6 +214,7 @@ func Main() {
 	obj.Flagcount("g", "debug code generation", &Debug['g'])
 	obj.Flagcount("h", "halt on error", &Debug['h'])
 	obj.Flagcount("i", "debug line number stack", &Debug['i'])
+	obj.Flagfn1("importmap", "add `definition` of the form source=actual to import map", addImportMap)
 	obj.Flagstr("installsuffix", "set pkg directory `suffix`", &flag_installsuffix)
 	obj.Flagcount("j", "debug runtime-initialized variables", &Debug['j'])
 	obj.Flagcount("l", "disable inlining", &Debug['l'])
@@ -501,6 +502,20 @@ func Main() {
 	Flusherrors()
 }
 
+var importMap = map[string]string{}
+
+func addImportMap(s string) {
+	if strings.Count(s, "=") != 1 {
+		log.Fatal("-importmap argument must be of the form source=actual")
+	}
+	i := strings.Index(s, "=")
+	source, actual := s[:i], s[i+1:]
+	if source == "" || actual == "" {
+		log.Fatal("-importmap argument must be of the form source=actual; source and actual must be non-empty")
+	}
+	importMap[source] = actual
+}
+
 func saveerrors() {
 	nsavederrors += nerrors
 	nerrors = 0
@@ -687,6 +702,11 @@ func importfile(f *Val, line int) {
 	}
 
 	path_ := f.U.(string)
+
+	if mapped, ok := importMap[path_]; ok {
+		path_ = mapped
+	}
+
 	if islocalname(path_) {
 		if path_[0] == '/' {
 			Yyerror("import path cannot be absolute path")