Fix first-time package fetches

Closes #19557
Closes #19561

Previously, `build.zig` was not being detected correctly by
`computeHash` for packages where there is a containing root directory.

Author: ianprime0509

src/Package/Fetch.zig

@@ -1447,7 +1447,7 @@ fn computeHash(
                 )),
             };
 
-            if (std.mem.eql(u8, entry.path, Package.build_zig_basename))
+            if (std.mem.eql(u8, entry_pkg_path, Package.build_zig_basename))
                 f.has_build_zig = true;
 
             const fs_path = try arena.dupe(u8, entry.path);