debian packaging: Move source package creation out of Makefile

...into separate script. Each time we run "make deb" we remove the
existing source package and build it from scratch (thanks to the
"debian-src-pkg/%/: FORCE" rule we had) so we don't actually gain
anything by using make's dependency tracking. Instead we lose a lot of
readibility because the order of operations isn't in linear order like
an imperative script is, and because the use of "%" and "$*" obscures
what the actual values are (compare
"stb-tester_$(VERSION)-%.debian.tar.xz" with
"stb-tester_$version-$release.debian.tar.xz").

I, at least, think this is clearer. I've also added a rule ("make
debsrc") to build all Ubuntu source packages before publishing them.

I also reordered the rules in the makefile so that the targets a user
would use (deb, debsrc, ppa-publish) come before the implementation
details.
debian packaging: Use the full dput target name

I suppose that just "ppa:stb-tester" worked, but now I've created
another PPA at "ppa:stb-tester/sandbox" (for testing my packaging
changes) and I want to be sure when publishing the official packages
they go to the right place. This is really just me being paranoid and
not knowing how "dput ppa:stb-tester" is going to behave.