Avoid double symlink in archive cache

[rjbou]

pro: have a cleaner cache by avoiding double symlinks (e.g. md5 -> sha512 & sha256 -> sha512 instead of md5 -> sha256 -> sha512)
cons: when the cache contains a sha256 -> md5 link, installing 2 packages that refer to the same archive but one with an md5 and the other with a sha256 ; the one with a sha256 won't be found in cache and redownloaded.

Another solution is to have opam always compute and store all checksums linked (from sha*'s), like proposed by @kit-ty-kate.

• add a test
• update master changes
• queued on Add every listed checksums of a given archive to the cache when fetching from it #6068