inline sequences to optimize away recursive calls to make/dh for implicit targets

inline sequences to optimize away recursive calls to make/dh for implicit targets

This assumes that all implicit rules file targets are owned by dh, so
it can just assume an implicit target can be optimized away to the commands
in its sequence.

I suppose this would break:

build:
	dh build

install: build
	dh install

binary-%: install
	my-binary-builder-$@

my-binary-builder-arch:
	echo "whee! I did something pointlessly complicated with make!"
	dh binary-arch
my-binary-builder-indep:
	dh binary-indep

But I can't imagine anyone does this, at least with a probability of 1 in
ten thousand, so hopefully it doesn't break any existing packages. :)