build: depend on $PKG-build instead of $PKG-files

Add a new target for build dependencies which is updated only once
during the build (as a final step). $PKG-files used to work like that,
but we will need to support multiple installation steps in the future,
so create a new empty file to indicate a completed build.
pkg_install: memorize install locations

Memorize install locations.
setup: add pkg_install_script

Add pkg_install_script to support creating package specific
installer scripts for the target location. These are created
under /usr/lib/pkg/install.d/, and the commands are supposed to be
embedded in build scripts under comments like:

	# install: echo Hello, world!