Currently we kinda assume that whatever is returned by `get_file_hash`
is of type self.PARAM_CHECKSUM, which is not actually true. E.g. for
http it might return `etag` or `md5`, but we don't distinguish between
those and call both `etag`. This is becoming more relevant for dir
hashes that are computed a few different ways (e.g. in-memory md5 or
upload to remote and get etag for the dir file).
Prerequisite for #4144 and #3069