fix: don't rewrite helm chart URLs pointing to a different host

When a helm chart index has download URLs on a different host than the
remote's base URL (e.g. charts hosted on intel.github.io but downloads
on github.com), the URL rewriting would produce broken proxy URLs.

Now URLs with a different host are left as-is so helm downloads
directly from the source. Same-host URLs are still rewritten through
the proxy as before.

Also fixes the route prefix to use /local/ for local members.

internal/virtual/merger.go

@@ -9,6 +9,7 @@ import (
 type MemberIndex struct {
 	RemoteName string
 	RepoType   models.RepoType
+	BaseURL    string
 	Body       []byte
 }