This gives people some flexibility when they need a path type, and
prevents a "combinatorial explosion" of various path stops.
I've re-implemented our existing `path` and `pathInStore` types using
`pathWith`. Our existing `package` type is potentially a candidate for
similar treatment, but it's a little quirkier (there's some stuff with
`builtins.hasContext` and `toDerivation` that I don't completely
understand), and I didn't want to muddy this PR with that.
As a happy side effect of this work, we get a new feature: the ability
to create a type for paths *not* in the store. This is useful for when a
module needs a path to a file, and wants to protect people from
accidentally leaking that file into the nix store.
