apk: add --dest-dir option

This allows splitting the apk files from the rootfs.

apk --root test --dest-dir rootfs add musl --initdb --allow-untrusted -X https://dl-cdn.alpinelinux.org/alpine/edge/main

will result in a database at test/lib/apk/db and the rootfs at test/rootfs.

I hope to add options for all paths. Then this will become a bit more useful.

Old MR with the same goal: !25 (closed)