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.cc

@@ -0,0 +1,72 @@
+#include "realisation.hh"
+#include "store-api.hh"
+
+namespace nix {
+
+MakeError(InvalidDerivationOutputId, Error);
+
+DrvOutput DrvOutput::parse(const std::string &strRep) {
+    const auto &[rawPath, outputs] = parsePathWithOutputs(strRep);
+    if (outputs.size() != 1)
+        throw InvalidDerivationOutputId("Invalid derivation output id %s", strRep);
+
+    return DrvOutput{
+        .drvPath = StorePath(rawPath),
+        .outputName = *outputs.begin(),
+    };
+}
+
+std::string DrvOutput::to_string() const {
+    return std::string(drvPath.to_string()) + "!" + outputName;
+}
+
+std::string Realisation::to_string() const {
+    std::string res;
+
+    res += "Id: " + id.to_string() + '\n';
+    res += "OutPath: " + std::string(outPath.to_string()) + '\n';
+
+    return res;
+}
+
+Realisation Realisation::parse(const std::string & s, const std::string & whence)
+{
+    // XXX: Copy-pasted from NarInfo::NarInfo. Should be factored out
+    auto corrupt = [&]() {
+        return Error("Drv output info file '%1%' is corrupt", whence);
+    };
+
+    std::optional<DrvOutput> id;
+    std::optional<StorePath> outPath;
+
+    size_t pos = 0;
+    while (pos < s.size()) {
+
+        size_t colon = s.find(':', pos);
+        if (colon == std::string::npos) throw corrupt();
+
+        std::string name(s, pos, colon - pos);
+
+        size_t eol = s.find('\n', colon + 2);
+        if (eol == std::string::npos) throw corrupt();
+
+        std::string value(s, colon + 2, eol - colon - 2);
+
+        if (name == "Id")
+            id = DrvOutput::parse(value);
+
+        if (name == "OutPath")
+            outPath = StorePath(value);
+
+        pos = eol + 1;
+    }
+
+    if (!outPath) corrupt();
+    if (!id) corrupt();
+    return Realisation {
+        .id = *id,
+        .outPath = *outPath,
+    };
+}
+
+} // namespace nix