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.
feat: Add cross-package-job and gexp compiler for it
This will allow compiling packages to cross targets, using gexps,
ungexping cross-package-job will produce a derivation for the given target.