Does the file packager need to simultanious embedding and preloading?

Supporting both at the same time complicates the code and I would hope this configuration is not necessary or useful.

We do have at least one test for this configuration: https://github.com/emscripten-core/emscripten/blob/ee5848479f9521bbb5b33c6d8c8f9158f1551ce8/test/test_other.py#L1951

This came up while reviewing #24802