When path mapping is enabled, `File` objects accessed in a
user-supplied callback passed to `Args#map_each` automatically have
their paths mapped.

Automatic rewriting is preferable to e.g. passing a rewriter object into
the callback: All paths emitted into command lines must be rewritten
with path mapping as inputs and outputs are staged at the mapped
locations, so the user would need to manually map all paths - there is
no choice. As an added benefit, the automatic rewriting ensures that
existing rules relying on `map_each` do not need to be modified to use
path mapping.