Allow loading object files from the command line in the external interpreter
#21035 (closed) provides the motivation in detail. The summary of it is that rules_haskell can arrange to execute TH splices without linking the library dependencies. Instead, object files are picked as needed from the dependencies and fed to GHC.
This feat, however, only works when using the internal interpreter. To fully support it, object files specified in the command line should also be sent to the external interpreter so it can use them when running splices.
#21620 is a ticket to document this feature.