This function returns and internalized dictionary from a package's metadata
plist file as specified by its arguments.
Update all code to use it where appropiate.
- There's no need to check rval for prop_dictionary_get_*, we are sure the
objects are there at prop_dictionary_set_* time.
- Avoid two chdir(2) calls per INSTALL/REMOVE run.
- Avoid using access(2) to check for existence of INSTALL/REMOVE scripts,
just try to run the executable directly and check for ENOENT.
