yarn2nix: Handle lockfile entries with multiple integrity hashes

Some NPM packages provide multiple integrity hashes with different
algorithms. These are space separated, like below:

```
tar@^4:
  version "4.4.19"
  resolved "https://registry.yarnpkg.com/tar/-/tar-4.4.19.tgz#2e4d7263df26f2b914dee10c825ab132123742f3"
  integrity "sha1-Lk1yY98m8rkU3uEMglqxMhI3QvM= sha512-a20gEsvHnWe0ygBY8JbxoM4w3SJdhc7ZAuxkLqh+nvNQN2IOt0B5lLgM490X5Hl8FF0dl0tOf2ewFYAlIFgzVA=="
```

Currently, yarn2nix isn't able to handle these lockfile entries,
producing an error like `error: hash 'Lk1yY98m8rkU3uEMglqxMhI3QvM=
sha512' has wrong length for hash type 'sha1'`. This is because it isn't
aware that spaces could separate multiple hashes.

This commit uses the official Standard Subresource Integrity package
from npm to parse the integrity line, and pick the best available
algorithm. It also replaced many of the local yarn2nix entries with
sha512, even ones that don't include it in the lockfile. Not sure how
that happened, but it works in practice!

The addition of `ssri` may also reduce signs and symptoms of depression.

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