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/store-api.hh

@@ -1,5 +1,6 @@
 #pragma once
 
+#include "realisation.hh"
 #include "path.hh"
 #include "hash.hh"
 #include "content-address.hh"
@@ -396,6 +397,8 @@ protected:
 
 public:
 
+    virtual std::optional<const Realisation> queryRealisation(const DrvOutput &) = 0;
+
     /* Queries the set of incoming FS references for a store path.
        The result is not cleared. */
     virtual void queryReferrers(const StorePath & path, StorePathSet & referrers)
@@ -468,6 +471,18 @@ public:
     virtual StorePath addTextToStore(const string & name, const string & s,
         const StorePathSet & references, RepairFlag repair = NoRepair) = 0;
 
+    /**
+     * Add a mapping indicating that `deriver!outputName` maps to the output path
+     * `output`.
+     *
+     * This is redundant for known-input-addressed and fixed-output derivations
+     * as this information is already present in the drv file, but necessary for
+     * floating-ca derivations and their dependencies as there's no way to
+     * retrieve this information otherwise.
+     */
+    virtual void registerDrvOutput(const Realisation & output)
+    { unsupported("registerDrvOutput"); }
+
     /* Write a NAR dump of a store path. */
     virtual void narFromPath(const StorePath & path, Sink & sink) = 0;