add support for arch-specific dependencies

[cjdcordeiro]

Atm, we support slice contents that are application specific, however, there's no way to specify an essential which is only required for a specific architecture.

Example: libnode108 only needs libatomic1 for riscv64.

[linostar]

Format suggestion for essential architecture-supported entry:
<slice>:<arch>

For example:
libatomic1_libs:riscv64

In case of a slice required for more than one architecture, a separate line is to be used for each, following the same format above.

[endersonmaia]

Maybe we could support some kind of negative rule, suppose that you want to exclude some arch.

For ex.: criu is not available for riscv64 so one would use criu:!riscv64 or even a list like criu:!{riscv64, amd64} (not a real scenario for criu here).

[dulmandakh]

I'm having same issue with canonical/chisel-releases#253. Chisel needs architecture support in dependencies

[HadrienPatte]

I'm getting hit by this issue in canonical/chisel-releases#442 and canonical/chisel-releases#441.

The libavcodec61/libavcodec60 and libavutil59/libavutil58 packages both depend on libvpl2 on amd64. And as this package does not exist on other architectures, the chisel-releases installation tests fail on other architectures with this error:

error: cannot find package "libvpl2" in archive(s)

The only way to currently get the CI to pass is to remove that dependency, but it means that the amd64 slices of those package are missing a dependency.

Similarly, on the same PRs, libavcodec61/libavcodec60 depend on librav1e0.7/librav1e0 which exists on every arch except i386, causing the i386 installability test to fail.