CBG-4322 use unique output files

[torcolvin]

CBG-4322 use unique output files

Written on top of #7351 code

In the case that there is a file with the same contents in multiple places, this fixes the idea that this writes this to the same file.

e.g. log_file_path in config.json is /var/log/sync_gateway

There is /var/log/sync_gateway/sg_info.log and /home/sync_gateway/logs/sg_info.log

The two will be concatenated together before this fix. There's another fix in #7351 which canonicalizes the paths with path.Pathlib.resolve before uploading.

I've considered whether to add some log messages to say where the files are picked up from, so we'd know why we got a duplicate. log is only written to stderr, but we could write tee to a file like sgcollect_info-output.log so we'd have an idea why things might go wrong.

New tests were written.