Don't use golang::* namespaces to avoid clashes with pygolang adding
something in there and getting compilation error due to conflict, when
e.g. nexedi/pygolang!17 lands.

-> use xgolang:: as top-level namespace for what was previously living
in golang:: shipped in wendelin.core.

Inside that xgolang:: namespace don't use the same package names that
might be used inside golang:: in pygolang. This avoids ambiguation and
compile error in the future on e.g. os::AfterFork - is `using namespace
golang` and `using namespace xgolang` were both activated.

-> Prefix all namespaces inside xgolang:: also with "x".