[hadd] Fix file equivalence test in case of remote files

[silverweed]

hadd is trying to use std::filesystem::equivalent indiscriminately for any source and target files, but the function only makes sense for local files.

This PR adds a function that checks if the files are remote and, if so, if simply compares their URL.

NOTE: this PR should be backported to 6.38.

Checklist:

• tested changes locally
• updated the docs (if necessary)

This PR fixes #20872

[dpiparo]

Thanks.