consider alternatives to the puppet mono-repo

another aspect of "how to publish our puppet repos" and how to collaborate is how to manage sub-repositories. expanding on the "mono-repo" problem discussed in legacy/trac#29387 (moved), i have found the following options:

1. current "monorepo" approach
2. pure librarian / r10k
3. git submodules
4. git subtree (originally from apenwarr but now merged in mainline since git 2.22)
5. git subrepo
6. myrepos and its numerous alternatives
7. Puppet itself, with the vcsrepo module or the puppet module command

i'll add more as i find them here. i should probably make a more detailed review of the advantages/inconvenients of all of those...