fix: respect host-system for native-inputs and target-inputs

fix: Support no target system

fix: Support plain packages in cross-shell native-inputs

feat: Use inputs from bags in inputs-from

To support arbitrary build system with its all build inputs, use bags
instead of packages for obtaining inputs.

feat: Add cross profile package support

Supports cross profile with the default target packages from
gnu-build-system. Accepts inputs, native-inputs or inputs-from where
the inputs are taken out of the package.

What remains is to think of a way to support arbitrary build system,
possibly via bags. As a first proof of concept, the build system could
be matched and packages hardcoded for each of them... then the inputs
can be taken when inputs-from is used, or one could make a list of
build systems to include in a new field.