Backport libfetchers from the flakes branch

This provides a pluggable mechanism for defining new fetchers. It adds
a builtin function 'fetchTree' that generalizes existing fetchers like
'fetchGit', 'fetchMercurial' and 'fetchTarball'. 'fetchTree' takes a
set of attributes, e.g.

  fetchTree {
    type = "git";
    url = "https://example.org/repo.git";
    ref = "some-branch";
    rev = "abcdef...";
  }

The existing fetchers are just wrappers around this. Note that the
input attributes to fetchTree are the same as flake input
specifications and flake lock file entries.

All fetchers share a common cache stored in
~/.cache/nix/fetcher-cache-v1.sqlite. This replaces the ad hoc caching
mechanisms in fetchGit and download.cc (e.g. ~/.cache/nix/{tarballs,git-revs*}).

This also adds support for Git worktrees (c169ea5904).

src/libfetchers/cache.hh

@@ -0,0 +1,34 @@
+#pragma once
+
+#include "fetchers.hh"
+
+namespace nix::fetchers {
+
+struct Cache
+{
+    virtual void add(
+        ref<Store> store,
+        const Attrs & inAttrs,
+        const Attrs & infoAttrs,
+        const StorePath & storePath,
+        bool immutable) = 0;
+
+    virtual std::optional<std::pair<Attrs, StorePath>> lookup(
+        ref<Store> store,
+        const Attrs & inAttrs) = 0;
+
+    struct Result
+    {
+        bool expired = false;
+        Attrs infoAttrs;
+        StorePath storePath;
+    };
+
+    virtual std::optional<Result> lookupExpired(
+        ref<Store> store,
+        const Attrs & inAttrs) = 0;
+};
+
+ref<Cache> getCache();
+
+}