Refactor packages (func name & UI) (#34773)

1. Use `OpenXxx` instead of `GetXxx` because the returned readers should
be correctly closed, and clarify the behaviors of the functions: they
increase the download counter
2. Use `packages-content` styles instead of `issue-content`

routers/api/packages/generic/generic.go

@@ -31,7 +31,7 @@ func apiError(ctx *context.Context, status int, obj any) {
 
 // DownloadPackageFile serves the specific generic package.
 func DownloadPackageFile(ctx *context.Context) {
-	s, u, pf, err := packages_service.GetFileStreamByPackageNameAndVersion(
+	s, u, pf, err := packages_service.OpenFileForDownloadByPackageNameAndVersion(
 		ctx,
 		&packages_service.PackageInfo{
 			Owner:       ctx.Package.Owner,