`ComposeRequest` included a `common.Distribution`, which had to be
resolved in PushComposeRequest. Use a real `distro.Distro` object here,
and push resolving it to the rcm package.
Change the `Distribution` on the (lower-case) `composeRequest` to a
string. This struct represents the incoming request. Since we're now
resolving the real distro object from the registry in the same function,
it seems redundant to validate the incoming distro twice.
c2c8fae093