expr: Have the `path()` constructor return unresolved[path] when validating for the host context

[mwiebe]

Description

In #373 (comment), it notes that when validating host-context expressions, the path format may be different from where it eventually runs. This can cause incorrect validation if we actually look at the path separators, for example.

Solution

If we change the path construction within expressions to return unresolved[path] whenever evaluating expressions before we know the OS, we can avoid that problem.l