Add ansible virtual repositories (#158)

Clients need one Galaxy server to install both internal collections and galaxy.ansible.com ones; ansible has no virtual type.

- merge member collection version lists in member order, the earlier member winning a duplicate version, cached with the shared virtual merge cache
- paginate limit/offset over the merged list, following remote member pages
- serve version detail from the owning member with href and download_url via the virtual; downloads 302 to the member through the member-redirect validation
- fail closed with 502 on a member error or a member versions list shorter than its count
- add docs/ansible/virtual.md and an e2e-docker case driving real ansible-galaxy

requires unkin/artifactapi#157

Reviewed-on: #158
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

README.md

@@ -27,7 +27,7 @@ Classification precedence, TTLs and eviction for every remote: [docs/common/cach
 Each type's paths, Terraform examples and local/virtual behaviour are in `docs/<type>/`:
 
 - `alpine`: [remote](docs/alpine/remote.md), [remote-github](docs/alpine/remote-github.md)
-- `ansible`: [remote](docs/ansible/remote.md), [local](docs/ansible/local.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)
 - `docker`: [remote](docs/docker/remote.md), [local](docs/docker/local.md)