I know! That's why I am asking. I have found surprisingly difficult to find a package that does this.
scrypt, with an output length that suits your use-case? (as Filippo points out)
Note: I wrote https://github.com/elithrar/simple-scrypt, which wraps Go's scrypt package and gives it a friendlier API (mimicking the bcrypt one). Handles salt generation for you and has sane default parameters, outputting a 32-byte derived key by default. e.g.
func deriveKey(key []byte) ([]byte, error) {
return scrypt.GenerateFromPassword([]byte("plaintext"), scrypt.DefaultParams)
}