Some pre/post-install scripts rely on them being open.
(It would be bette to leave them open and show the output, but I didn't
want to change the behaviour)
Fixed pkgng_exists explorer (can't reference $__global)
Fixed logic in __package/pkgng_freebsd/gencode-remote
Wasn't installing package if it already exists, but after explorer execution
Create __package_pkgng_freebsd type to handle package management via pkg-ng
Add requirement generation in __package/manifest
Add explorer inside __package to find if pkg-ng is installed on the target