core: INCOMPATIBLE CHANGE: Split archive files in two parts (meta and content)

core: INCOMPATIBLE CHANGE: Split archive files in two parts (meta and content)

This will allow us to have hardlink checkouts of archives.  A key use
case here is an archive repo of an OS (with root-owned files etc.)
where we want to do builds in a user tree.

A positive side effect of doing things this way is that now the SHA256
checksums for a given file should be identical regardless of whether
it's stored in an archive or bare repository.