Unified fetcher caching system

src/libexpr/common-eval-args.cc

src/libexpr/flake/flake.cc

src/libexpr/flake/flakeref.cc

src/libexpr/flake/flakeref.hh

src/libexpr/primops/fetchTree.cc

src/libstore/fetchers/attrs.cc

src/libstore/fetchers/attrs.hh

src/libstore/fetchers/cache.cc

src/libstore/fetchers/cache.hh

src/libstore/fetchers/fetchers.cc

src/libstore/fetchers/fetchers.hh

src/libstore/fetchers/git.cc

src/libstore/fetchers/github.cc

src/libstore/fetchers/indirect.cc

src/libstore/fetchers/mercurial.cc

src/libstore/fetchers/registry.cc

src/libstore/fetchers/registry.hh

src/libstore/fetchers/tarball.cc

src/nix/flake.cc

tests/fetchMercurial.sh