Instead of having to manually stub packages that should not be
downloaded we instead automatically stub all packages (except a small
list of whitelisted ones). Tests can re-introduce the real package by
using the `realPkgs` module argument.
Nushell has the option to source from the login.nu file in the case
that nushell is used as a login shell. This commit adds the login file
alongside the existing config and env files as another configuration
option.
Previously the nushell module did not differentiate between Linux and
Darwin when deciding where to place config files, whereas nushell
does. This commit fixes that.
