go bindings: move to non github.com url

[cyphar]

Since we are a new Go package, it might be a good idea to make the import path cyphar.com/pathrs or something, rather than github.com/cyphar/libpathrs. Unfortunately, Go modules have made import paths Very Important (TM) and basically impossible to change once you have users. I don't particularly like the idea of enshrining github.com/cyphar for all eternity...