Can't find package.json

[binarykitchen]

When you define an external rslib config file with the -c parameter, it can't find the package.json anymore. For example:

rslib build -c ./etc/rsbuild/config.ts

It will complain that it can't find the package.json from the root folder.

[fi3ework]

I believe it would be a bug, and we didn't cover that use case in out test. let's track that in #361.