Previously a particular package could only ever be installed to target
or native. Instead, allow any package to be installed to either, with
target being the default. Native packages can only ever have native
dependencies, but target packages can depend on native ones in order to
install tools which they need for their build process.
We need to distinguish between tools we compile to use as part of the
build process, and things we build for the eventual target. Install
these into separate install directories; for now, pkg-config is the
only one of these.
