The `API.arch` member was (mostly) used to read the name of the
architecture.
The only non-name use was for the purposes of reading RPM repositories
from the configuration, in `reporegistry.ReposByArch()`, a thin wrapper
around `reporegistry.ReposByArchName()`.
Removing the `arch` member from the API and using the new `archName`
that is set up in the API constructor lets us control the arch name that
is set without relying on a valid `distro.Arch` object being available
(which would depend on having a valid `distro.Distro` object).
Replaced all calls to `ReposByArch()` with `ReposByArchName()` which
depends on the arch and distro name strings instead of a full
`distro.Arch`.
baa2339de4