dak make-pkg-file-mapping: do not include sources from other archives

dak make-pkg-file-mapping: do not include sources from other archives

Include only information about a source package's files if its .dsc is
in the requested archive.

Previously when xy_1.0-1 was in the requested archive and xy_1.0-2
using the same upstream tarball in a different archive, dak would
still include information about xy_1.0-2's upstream tarball resulting
in confusing entries on snapshot.d.o.

Reported-by: Salvatore Bonaccorso <carnil@debian.org>