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/Module.zig

@@ -3185,6 +3185,9 @@ pub fn importFile(
     if (cur_file.pkg.table.get(import_string)) |pkg| {
         return mod.importPkg(pkg);
     }
+    if (!mem.endsWith(u8, import_string, ".zig")) {
+        return error.PackageNotFound;
+    }
     const gpa = mod.gpa;
 
     // The resolved path is used as the key in the import table, to detect if