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/libstore/store-api.cc

@@ -6,6 +6,7 @@
 #include "thread-pool.hh"
 #include "json.hh"
 #include "derivations.hh"
+#include "url.hh"
 
 #include <future>
 
@@ -40,7 +41,7 @@ Path Store::followLinksToStore(std::string_view _path) const
         path = absPath(target, dirOf(path));
     }
     if (!isInStore(path))
-        throw Error(format("path '%1%' is not in the Nix store") % path);
+        throw NotInStore("path '%1%' is not in the Nix store", path);
     return path;
 }
 
@@ -866,27 +867,7 @@ std::pair<std::string, Store::Params> splitUriAndParams(const std::string & uri_
     Store::Params params;
     auto q = uri.find('?');
     if (q != std::string::npos) {
-        for (auto s : tokenizeString<Strings>(uri.substr(q + 1), "&")) {
-            auto e = s.find('=');
-            if (e != std::string::npos) {
-                auto value = s.substr(e + 1);
-                std::string decoded;
-                for (size_t i = 0; i < value.size(); ) {
-                    if (value[i] == '%') {
-                        if (i + 2 >= value.size())
-                            throw Error("invalid URI parameter '%s'", value);
-                        try {
-                            decoded += std::stoul(std::string(value, i + 1, 2), 0, 16);
-                            i += 3;
-                        } catch (...) {
-                            throw Error("invalid URI parameter '%s'", value);
-                        }
-                    } else
-                        decoded += value[i++];
-                }
-                params[s.substr(0, e)] = decoded;
-            }
-        }
+        params = decodeQuery(uri.substr(q + 1));
         uri = uri_.substr(0, q);
     }
     return {uri, params};