Add deb virtual repositories (#138)

Virtual repos merge helm, pypi, rpm, npm, gems, ansible, goproxy and puppet but not deb, so several apt repos (e.g. github_deb remotes plus a local deb) cannot be served as one apt source.

- merge member Packages for flat (`./`) and `dists/<suite>` layouts; member order wins duplicate (Package, Version, Architecture)
- generate an unsigned Release with Acquire-By-Hash, serving indexes by name and by SHA256 from both retained generations
- prefix Filename with the member name and 302 downloads to the member route
- cache merges per suite on the shared virtual cache, capped at 32 suites per virtual (LRU)
- add docs/deb/virtual.md, unit tests and dockerised apt e2e for flat and suite layouts

Reviewed-on: #138
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

README.md

@@ -29,7 +29,7 @@ Each type's paths, Terraform examples and local/virtual behaviour are in `docs/<
 - `alpine`: [remote](docs/alpine/remote.md), [remote-github](docs/alpine/remote-github.md)
 - `ansible`: [remote](docs/ansible/remote.md), [local](docs/ansible/local.md), [virtual](docs/ansible/virtual.md)
 - `cargo`: [remote](docs/cargo/remote.md)
-- `deb`: [remote](docs/deb/remote.md), [remote-github](docs/deb/remote-github.md)
+- `deb`: [remote](docs/deb/remote.md), [remote-github](docs/deb/remote-github.md), [virtual](docs/deb/virtual.md)
 - `docker`: [remote](docs/docker/remote.md), [local](docs/docker/local.md)
 - `generic`: [remote](docs/generic/remote.md)
 - `gems`: [remote](docs/gems/remote.md), [local](docs/gems/local.md), [virtual](docs/gems/virtual.md)