Rename binary package to php-swiftmailer

The package does not follow the pkg-php name convention, thus forcing other
packages using dh_phpcomposer to provide a debian/pkg-php-tools-overrides
in order to generate working dependencies out of a composer.json.
Therefore the binary package is renamed to php-swiftmailer, providing a
transitional package libphp-swiftmailer.
Update Vcs fields and accept package rename patch

Thanks: Daniel Beyer <dabe@deb.ymc.ch>
Closes: #757197
Update debian/watch for new upstream location