Refactor LfsImportService and ImportService

What does this MR do?

In https://gitlab.com/gitlab-org/gitlab-ee/issues/10871 we are going to reuse the service that import Lfs Objects, but ImportService has too much logic about it. In this MR we refactor the code to make it more straightforward and reusable.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

app/services/projects/lfs_pointers/lfs_download_link_list_service.rb

@@ -21,9 +21,9 @@ def initialize(project, remote_uri: nil)
       # This method accepts two parameters:
       # - oids: hash of oids to query. The structure is { lfs_file_oid => lfs_file_size }
       #
-      # Returns a hash with the structure { lfs_file_oids => download_link }
+      # Returns a array of LfsDownloadObject
       def execute(oids)
-        return {} unless project&.lfs_enabled? && remote_uri && oids.present?
+        return [] unless project&.lfs_enabled? && remote_uri && oids.present?
 
         get_download_links(oids)
       end