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/realisation.hh

@@ -0,0 +1,34 @@
+#pragma once
+
+#include "path.hh"
+
+namespace nix {
+
+struct DrvOutput {
+    StorePath drvPath;
+    std::string outputName;
+
+    std::string to_string() const;
+
+    static DrvOutput parse(const std::string &);
+
+    bool operator<(const DrvOutput& other) const { return to_pair() < other.to_pair(); }
+    bool operator==(const DrvOutput& other) const { return to_pair() == other.to_pair(); }
+
+private:
+    // Just to make comparison operators easier to write
+    std::pair<StorePath, std::string> to_pair() const
+    { return std::make_pair(drvPath, outputName); }
+};
+
+struct Realisation {
+    DrvOutput id;
+    StorePath outPath;
+
+    std::string to_string() const;
+    static Realisation parse(const std::string & s, const std::string & whence);
+};
+
+typedef std::map<DrvOutput, Realisation> DrvOutputs;
+
+}