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/libexpr/primops.hh

@@ -20,6 +20,7 @@ struct RegisterPrimOp
    them. */
 /* Load a ValueInitializer from a DSO and return whatever it initializes */
 void prim_importNative(EvalState & state, const Pos & pos, Value * * args, Value & v);
+
 /* Execute a program and parse its output */
 void prim_exec(EvalState & state, const Pos & pos, Value * * args, Value & v);