#1835 subtly changed the semantics of the `LoadFile` plugin hook to no
longer have the current script location available for signature files
being loaded through `@load-sigs`. This was undocumented behavior, so
it's technically not a regression, but since at least one external
plugin is depending on it, this change restores the old behavior.
This (1) fixes an issue where signature files supplied on the command
line wouldn't pass through the hooks, and (2) prepares for allowing
hooks to supply the content of a signature file directly.