* scripts/Dpkg/Source/Package/V1_0/native.pm: New module that
handles native source packages with any compression.
* scripts/Dpkg/Source/Package/V1_0.pm: Adjusted to use the previous
module when it comes to handle native packages. Simplified
several other code paths.
* scripts/Makefile.am, scripts/po/POTFILES.in: Add the new module.
* scripts/dpkg-source.pl: Add the new format as fallback to try
when building a source package. It will likely only be selected
when non-gzip compression has been requested.