gc: more precise handling of import .

Fixes #455.

R=ken2
CC=golang-dev
https://golang.org/cl/186212

src/cmd/gc/dcl.c

@@ -149,6 +149,11 @@ testdclstack(void)
 void
 redeclare(Sym *s, char *where)
 {
+	if(s->lastlineno == 0)
+		yyerror("%S redeclared %s\n"
+			"\tprevious declaration during import",
+			s, where);
+	else
 		yyerror("%S redeclared %s\n"
 			"\tprevious declaration at %L",
 			s, where, s->lastlineno);

src/cmd/gc/lex.c

@@ -1470,9 +1470,9 @@ mkpackage(char* pkg)
 				if(s->def->op == OPACK) {
 					// throw away top-level package name leftover
 					// from previous file.
-					// TODO(rsc): remember that there was a package
-					// name, so that the name cannot be redeclared
-					// as a non-package in other files.
+					// leave s->block set to cause redeclaration
+					// errors if a conflicting top-level name is
+					// introduced by a different file.
 					if(!s->def->used && !nsyntaxerrors)
 						yyerrorl(s->def->lineno, "imported and not used: %s", s->def->sym->name);
 					s->def = N;

src/cmd/gc/subr.c

@@ -353,6 +353,7 @@ importdot(Sym *opkg, Node *pack)
 				continue;
 			}
 			s1->def = s->def;
+			s1->block = s->block;
 			s1->def->pack = pack;
 			n++;
 		}