fix run-time error reporting for variables in a submodule

Error reports used the "source" field of a module, which
doesn't have submodule information, or the "name" field of
a module, which might not match an actual filename (".ss"
vs. ".rkt"). Create the right combination.
fix `rename-transformer-target` for chaperoned structs
fix "props" for "pict-snip-pkgs"