Dpkg::Source::Archive/Package: misc fixes and changes

Dpkg::Source::Archive/Package: misc fixes and changes

* scripts/Dpkg/Source/Archive.pm: Replace unsafe rmtree() by
the safer erasedir() function.
* scripts/Dpkg/Source/Archive.pm (add_file, add_directory): Prefix
filename with chdir when the directory of the tar process isn't
the current directory.

* scripts/Dpkg/Source/Package.pm (find_original_tarballs): New
function to find original tarballs possibly associated to the current
source package.
* scripts/Dpkg/Source/Package.pm (add_file): Support adding file
who are not stored in the current directory (or who are prefixed with
"./").
* scripts/Dpkg/Source/Package.pm (can_build): New function to check
if a source package build is likely to succeed given the constraints
of a particular source package format. Will be used by dpkg-source to
find an appropriate format.