make DerivationOutput explicitely generic

It is already parametrized, but `name` field is too specific
to accommodate i.e. `DerivationOutput StorePath` which is
used by `Derivation` type. So we call it `output` instead
and turn the type variable to just `a`.

So
* for `Realisation`s this is `DerivationOutput OutputName`
* for `Derivation`s this is `DerivatonOutput StorePath`
* for content addressed derivations this might be `DerivationOutput Void` as the path isn't known ahead of time.

So only its shape is important.

Related to https://github.com/Gabriella439/Haskell-Nix-Derivation-Library/pull/24

hnix-store-core/src/System/Nix/Realisation.hs

@@ -28,11 +28,11 @@ import qualified Data.Text.Lazy.Builder
 import qualified System.Nix.Hash
 
 -- | Output of the derivation
-data DerivationOutput outputName = DerivationOutput
+data DerivationOutput a = DerivationOutput
   { derivationOutputHash :: DSum HashAlgo Digest
   -- ^ Hash modulo of the derivation
-  , derivationOutputName :: outputName
-  -- ^ Name of the output
+  , derivationOutputOutput :: a
+  -- ^ Output (either a OutputName or StorePatH)
   } deriving (Eq, Generic, Ord, Show)
 
 data DerivationOutputError
@@ -74,7 +74,7 @@ derivationOutputBuilder
 derivationOutputBuilder outputName DerivationOutput{..} =
      System.Nix.Hash.algoDigestBuilder derivationOutputHash
   <> Data.Text.Lazy.Builder.singleton '!'
-  <> Data.Text.Lazy.Builder.fromText (outputName derivationOutputName)
+  <> Data.Text.Lazy.Builder.fromText (outputName derivationOutputOutput)
 
 -- | Build realisation context
 --

hnix-store-json/tests/JSONSpec.hs

@@ -38,7 +38,7 @@ sampleDerivationOutput = DerivationOutput
       $ System.Nix.Hash.mkNamedDigest
           "sha256"
           "1b4sb93wp679q4zx9k1ignby1yna3z7c4c2ri3wphylbc2dwsys0"
-  , derivationOutputName =
+  , derivationOutputOutput =
       forceRight
       $ System.Nix.OutputName.mkOutputName "foo"
   }