Store metadata about drv outputs realisations

For each known realisation, store:
- its output
- its output path

This comes with a set of needed changes:

- New `realisations` module declaring the types needed for describing
  these mappings
- New `Store::registerDrvOutput` method registering all the needed informations
  about a derivation output (also replaces `LocalStore::linkDeriverToPath`)
- new `Store::queryRealisation` method to retrieve the informations for a
  derivations

This introcudes some redundancy on the remote-store side between
`wopQueryDerivationOutputMap` and `wopQueryRealisation`.
However we might need to keep both (regardless of backwards compat)
because we sometimes need to get some infos for all the outputs of a
derivation (where `wopQueryDerivationOutputMap` is handy), but all the
stores can't implement it − because listing all the outputs of a
derivation isn't really possible for binary caches where the server
doesn't allow to list a directory.

src/libstore/remote-store.hh

@@ -81,6 +81,10 @@ public:
     StorePath addTextToStore(const string & name, const string & s,
         const StorePathSet & references, RepairFlag repair) override;
 
+    void registerDrvOutput(const Realisation & info) override;
+
+    std::optional<const Realisation> queryRealisation(const DrvOutput &) override;
+
     void buildPaths(const std::vector<StorePathWithOutputs> & paths, BuildMode buildMode) override;
 
     BuildResult buildDerivation(const StorePath & drvPath, const BasicDerivation & drv,