data: make repositories available via go:embed

This commit allows us to discover/load repositories via `go:embed`.
Note that unlike the `testrepos.go` code this one does not provide
a `New()` helper. Instead we will modify the `RepoRegistry` to
look for the search paths for repo configuration first and then
fallback to the buildin repos (unless disabled via a flag).

Embedding also allows us an easy transition path in composer. The
rpm specfile will simply need to copy the repository data from
`vendor/github.com/osbuild/images/data/repositories` instead of
the current location and we can delete the repositories already
covered here (composer will still carry the `-no-aux-keys` repos
and the symlinks (which sadly cannot be expressed via go:embed)).

data/repositories/repos.go

@@ -0,0 +1,8 @@
+package repos
+
+import (
+	"embed"
+)
+
+//go:embed *.json
+var FS embed.FS