ImportLibrary: read from each used file's mapped ID

The vast majority of the time, the mapped ID will be identical to the
path returned by getUsedFiles().  For Flex datasets with server mapping
enabled client-side, however, the mapped ID will be different to allow
all files to end up in the same directory.  This eliminates the need for
server-side server mappings.

See ticket #11781.
Merge pull request #1867 from melissalinkert/ticket-11781

ImportLibrary: read from each used file's mapped ID