reporegistry: do not prepend repositories in confPaths

[mvo5]

[draft as this has too few tests, but I want to get feedback
before writing them]

This PR changes the API for reporegistry in two incompatible
ways. The important change is that we want to drop the
prepending of repositories when loading from confPaths.

Doing this on its own has the disadvantage of breaking
the API without breaking the builds. So this includes a
small tweak to allow adding repos via fs.FS directly
which will mean the code in ibcli will become simpler:

diff --git a/cmd/image-builder/repos.go b/cmd/image-builder/repos.go
index bc238dc..5d16a95 100644
--- a/cmd/image-builder/repos.go
+++ b/cmd/image-builder/repos.go
@@ -2,8 +2,6 @@ package main
 
 import (
        "io/fs"
-       "os"
-       "path/filepath"
 
        "github.com/osbuild/images/data/repositories"
        "github.com/osbuild/images/pkg/reporegistry"
@@ -27,17 +25,6 @@ var newRepoRegistry = func(dataDir string) (*reporegistry.RepoRegistry, error) {
                dataDirs = defaultDataDirs
        }
 
-       // XXX: think about sharing this with reporegistry?
-       var fses []fs.FS
-       for _, d := range dataDirs {
-               fses = append(fses, os.DirFS(filepath.Join(d, "repositories")))
-       }
-       fses = append(fses, repos.FS)
-
        // XXX: should we support disabling the build-ins somehow?
-       conf, err := reporegistry.LoadAllRepositoriesFromFS(fses)
-       if err != nil {
-               return nil, err
-       }
-       return reporegistry.NewFromDistrosRepoConfigs(conf), nil
+       return reporegistry.New(dataDirs, []fs.FS{repos.FS})
 }

And for composer it will just be adding a nil to the New()
call.

Note that this also allows us to unexport the LoadAllRepositories
repository loading as this is only used by composer or ibcli.

---

reporegistry: do not prepend repositories in confPaths

This commit breaks the API by changing the confPaths loading
in reporegistry to stop prepending repositories/. This makes
it easier to use this interface in e.g. cmd/build.

Note that this requires changes in the upstream projecs, notably
osbuild-composer and image-builder-cli.

---

This commit changes reporegistry.New() to take a new parameter
repoConfigFS []fs.FS. With that we can unexport the public
LoadAllRepositories{,FromFS} as the only user for those is
composer and that can simply use .New().

[thozza]

The idea of the change looks good to me, but the code needs some fixes for tests to pass...