Go packages by Fox.ONE

Before this change, we put a single go.mod file in the root directory, any project that import the library will have a lot of unnecessary dependencies.

Then we delete the global go.mod file and put go.mod in each sub-package directory, based on this, there are some points that need attention here,

1. Migrate from old way, remove the github.com/fox-one/pkg line in your go.mod file, then execute go mod tidy will load all sub-package you imported.
2. No more tags like vX.Y.Z will be created later, according to the go mod doc, use sub-package/vX.Y.Z instead.
3. When you add a new sub-package, make sure that your sub-package does not depend on github.com/fox-one/pkg, otherwise it will lead to dependency conflict for the user.