`hashPrefix` not applied when using default scope name

[susnux]

When using the default scope name the hashPrefix option is ignored, see here:

https://github.com/madyankin/postcss-modules/blob/v6.0.1/src/scoping.js#L39

This happens because hashPrefix is used when generating the hash, but when the default scope names are used this package calculates the hash on its own.
Meaning the hashPrefix is only used when genericNames is generating the hashes but for default names this package generates the hash without the prefix on its own.