stage2: slightly improve error reporting for missing imports

There is now a distinction between `@import` with a .zig extension and
without. Without a .zig extension it assumes it is a package name, and
returns error.PackageNotFound if not mapped into the package table.

src/codegen/x86_64.zig

@@ -4,7 +4,6 @@ const mem = std.mem;
 const assert = std.debug.assert;
 const ArrayList = std.ArrayList;
 const Allocator = std.mem.Allocator;
-const Type = @import("../Type.zig");
 const DW = std.dwarf;
 
 // zig fmt: off