gc, ld: package name main no longer reserved

R=ken2
CC=golang-dev
https://golang.org/cl/4128054

src/cmd/gc/go.y

@@ -242,14 +242,6 @@ import_package:
 		
 		if(safemode && !curio.importsafe)
 			yyerror("cannot import unsafe package %Z", importpkg->path);
-
-		// NOTE(rsc): This is no longer a technical restriction:
-		// the 6g tool chain would work just fine without giving
-		// special meaning to a package being named main.
-		// Other implementations might need the restriction
-		// (gccgo does), so it stays in the language and the compiler.
-		if(strcmp($2->name, "main") == 0)
-			yyerror("cannot import package main");
 	}
 
 import_safety:

src/cmd/gc/lex.c

@@ -423,6 +423,15 @@ importfile(Val *f, int line)
 		errorexit();
 	}
 
+	// The package name main is no longer reserved,
+	// but we reserve the import path "main" to identify
+	// the main package, just as we reserve the import 
+	// path "math" to identify the standard math package.
+	if(strcmp(f->u.sval->s, "main") == 0) {
+		yyerror("cannot import \"main\"");
+		errorexit();
+	}
+
 	if(strcmp(f->u.sval->s, "unsafe") == 0) {
 		if(safemode) {
 			yyerror("cannot import package unsafe");

src/cmd/ld/go.c

@@ -148,8 +148,6 @@ ldpkg(Biobuf *f, char *pkg, int64 len, char *filename, int whence)
 		}
 		if(strcmp(pkg, "main") == 0 && strcmp(name, "main") != 0)
 			fprint(2, "%s: %s: not package main (package %s)\n", argv0, filename, name);
-		else if(strcmp(pkg, "main") != 0 && strcmp(name, "main") == 0)
-			fprint(2, "%s: %s: importing %s, found package main", argv0, filename, pkg);
 		loadpkgdata(filename, pkg, p0, p1 - p0);
 	}