fix: don't rewrite helm chart URLs pointing to a different host (#56)

## Problem
Helm charts like Intel device plugins have download URLs on `github.com` but the chart index is served from `intel.github.io`. The merger rewrites all URLs through the proxy, constructing:
```
https://artifactapi/api/v1/remote/intel-helm/intel/helm-charts/releases/download/...
```
Which proxies to `https://intel.github.io/helm-charts/intel/helm-charts/releases/download/...` — a 404.

## Fix
Compare the download URL host against the remote's base URL host. If they differ, leave the URL as-is so helm downloads directly from the source. Same-host URLs are still rewritten through the proxy.

Also adds `BaseURL` to `MemberIndex` so the merger has the context it needs, and uses the correct `/local/` vs `/remote/` route prefix.

Reviewed-on: #56
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

internal/virtual/engine.go

@@ -79,7 +79,7 @@ func (e *Engine) fetchMemberIndexes(ctx context.Context, virt models.Virtual, pa
 					results[idx] = result{err: fmt.Errorf("local index %q: %w", name, err)}
 					return
 				}
-				results[idx] = result{index: MemberIndex{RemoteName: name, RepoType: remote.RepoType, Body: body}}
+				results[idx] = result{index: MemberIndex{RemoteName: name, RepoType: remote.RepoType, BaseURL: remote.BaseURL, Body: body}}
 				return
 			}
 
@@ -102,7 +102,7 @@ func (e *Engine) fetchMemberIndexes(ctx context.Context, virt models.Virtual, pa
 				return
 			}
 
-			results[idx] = result{index: MemberIndex{RemoteName: name, RepoType: remote.RepoType, Body: body}}
+			results[idx] = result{index: MemberIndex{RemoteName: name, RepoType: remote.RepoType, BaseURL: remote.BaseURL, Body: body}}
 		}(i, memberName)
 	}