Previously, the script was caching any file already present in the
destination directory, regardless of its origin. This caused issues
when the directory contained files copied from external sources like
`autoreconf --install`.

For example:
1. `./autogen.sh --install` copies `config.guess` and `config.sub`
   from the system to `./tool`.
2. `ruby tool/downloader.rb -d tool -e gnu config.guess config.sub`
   treats those files as if they were downloaded and caches them.
3. Removing the files: `rm tool/config.guess tool/config.sub`.
4. Running the downloader again, it mistakenly restores the cached
   files instead of downloading fresh versions.