Create Dpkg::Source::CompressedFile as spinoff of Dpkg::Source::Compressor

Create Dpkg::Source::CompressedFile as spinoff of Dpkg::Source::Compressor

* scripts/Dpkg/Source/Compressor.pm: Drom from this object
all filename related code and move it ...
* scripts/Dpkg/Source/CompressedFile.pm: ...here. This
object is a named file and it handles either explicit
compression (call to set_compression()) or implicit compression
(compression type is guessed from the filename). It offers
open_for_write() and open_for_read() to write into/read from
a data stream that is compressed/uncompressed on the fly.
* scripts/Dpkg/Source/Archiver.pm: Make it based on
CompressedFile and simplify code.
* scripts/dpkg-source.pl: Adapt code to the new interface of
the Compressor object.
* scripts/Makefile.am, scripts/po/POTFILES.in: register new
file CompressedFile.pm.