Make package_build build-dependency-aware

[matteodelabre]

The toltec-dev/toltec#274 PR added build-time dependencies and updated the repo_build script so that the dependencies of a recipe are always built before that recipe. However, it did not update the package_build script. As a consequence, users have to manually handle dependencies when building individual packages. To see the issue:

• Try to build the zshelf package from an empty build directory. You’ll get an error saying that the /repo/Packages.gz file does not exist. This can be fixed by creating an empty Packages.gz file in the build/repo folder.
• Even with the empty Packages.gz created, try to build the recrossable recipe. You’ll get an error saying that the libdlib cannot be found. This can be fixed by first building the libdlib recipe.

A better behavior would be for the package_build script to trigger the build of needed packages automatically when they are missing and to always create the packages index before running the build.

[matteodelabre]

Since we’ve moved the build system in a separate project we need to rethink a little bit how we may solve this. We can’t simply “trigger the build of needed packages automatically when they are missing”, because the build script doesn’t know where all the recipe sources are located anymore.

A possible solution would be to generate a helpful error message, saying that one of the dependencies are missing for the build, clearly indicating which package is missing, and indicating what steps need to be taken to fix this.

[Eeems]

I think it would make sense to allow passing in multiple package directories to build at once, and have all the dependency handling that the toltec build system had implemented. That or have some extra glue around that in toltec and just manually call the toltecmk code from there.