Fix partial download recovery and handle missing Files field in Sources

[dybai]

This commit addresses two issues:

1. Fix partial download recovery:

   • When network errors occur during download, some files in skel_path may be smaller than expected. Previously, apt-mirror would not re-download these incomplete files on subsequent runs, requiring manual deletion. Now, the script automatically detects and removes incomplete temporary files before checking if an update is needed.

2. Handle missing Files field in Sources index:

   • Some source package index files don't contain a Files: field, which caused "Use of uninitialized value" warnings. While these warnings didn't affect synchronization, they were noisy. The code now checks for the existence of the Files: field before processing it, only handling entries that contain this field.

Changes:

• Add check_and_clean_partial_download() function to detect and remove incomplete downloads in skel_path
• Call check_and_clean_partial_download() before need_update() for both Packages and Sources processing
• Add existence check for Files: field in Sources index processing