This MR adds a new index field (file-name, apkv2 n) which contains the filename relative to the parent directory of the repository index.
These fields are mapped to apk_package.remote_filename. If apk_package.remote_filename is set, then it will be used in place of the inferred package filename when generating URIs.
While here, refactor the URI generation logic to make it more clear.
Closes #10816.