cmd/gc: don't import the same package multiple times.

Implementation suggested by DMorsing.

R=golang-dev, dave, daniel.morsing, rsc
CC=golang-dev
https://golang.org/cl/6903059

src/cmd/gc/go.h

@@ -388,6 +388,7 @@ struct	Pkg
 	Sym*	pathsym;
 	char*	prefix;		// escaped path for use in symbol table
 	Pkg*	link;
+	uchar	imported;	// export data of this package was parsed
 	char	exported;	// import line written in export data
 	char	direct;	// imported directly
 };

src/cmd/gc/lex.c

@@ -690,6 +690,16 @@ importfile(Val *f, int line)
 	}
 	importpkg = mkpkg(path);
 
+	// If we already saw that package, feed a dummy statement
+	// to the lexer to avoid parsing export data twice.
+	if(importpkg->imported) {
+		file = strdup(namebuf);
+		p = smprint("package %s\n$$\n", importpkg->name);
+		cannedimports(file, p);
+		return;
+	}
+	importpkg->imported = 1;
+
 	imp = Bopen(namebuf, OREAD);
 	if(imp == nil) {
 		yyerror("can't open import: \"%Z\": %r", f->u.sval);