flakes: search up to git or filesystem boundary

While parsing a flakeref, upon not finding a flake.nix, search upwards
until git or filesystem boundary.

tests/flake-searching.sh

@@ -0,0 +1,24 @@
+source common.sh
+
+clearStore
+
+cp ./simple.nix ./simple.builder.sh ./config.nix $TEST_HOME
+cd $TEST_HOME
+cat <<EOF > flake.nix
+{
+    outputs = a: {
+       defaultPackage.$system = import ./simple.nix;
+       packages.$system.test = import ./simple.nix;
+    };
+}
+EOF
+mkdir subdir
+cd subdir
+
+for i in "" . "$PWD" .# .#test; do
+    nix build $i || fail "flake should be found by searching up directories"
+done
+
+for i in "path:$PWD"; do
+    ! nix build $i || fail "flake should not search up directories when using 'path:'"
+done