Ensure icon and splash paths on export

[Summersay415]

Fixes #100919

[bruvzg]

Tested export of PCKs and ZIPs, working as expected.

[bruvzg]

Note: it seems like files were actually exported, but with uid:// prefix in the names:

PCK file listing

ZIP file listing

This is outside the scope of this PR, but we might want to add some checks (or ResourceUID::ensure_path calls) to the PCK/ZIP writing code to catch a similar issue.

[Repiteo]

Thanks!