submission_package: Move ExeFS and RomFS initialization to its own function

Like the other two bits of factored out code, this can also be put
within its own function. We can also modify the code so that it accepts
a const reference to a std::vector of files, this way, we can
deduplicate the file retrieval.

Now the constructor for NSP isn't a combination of multiple behaviors in
one spot. It's nice and separate.