Move the compression record to a dedicated module so that it can be used
outside (guix scripts pack) module.
* guix/scripts/pack.scm (<compressor>, %compressors, lookup-compressor): Move
it to ...
* gnu/compression.scm: ... this new file.
* gnu/ci.scm: Adapt it.
* local.mk (GNU_SYSTEM_MODULES): Add it.
