dpkg-source: use a list of possible format for building a source package

dpkg-source: use a list of possible format for building a source package

* scripts/dpkg-source.pl: Define a list of formats that can be used
to build a source package and use the first one that is able to build
a source package. Let the user add formats to try at the beginning of that
list with the --format=<format> command-line option.
* scripts/Dpkg/Source/Package/V1_0.pm (can_build): Only accept to build a
source package is the compression option is gzip.
* scripts/Dpkg/Source/Package.pm (can_build): Return a proper error
message.
* scripts/dpkg-source.pl: Use the info() function to report progress.