Try and help if a file is missing at eval time

When evaluating a local git flake, if a file a missing at eval-time but
present in the source directory, tell the user that he probably forgot
to check it out

Fix #4507

tests/flake-missing-file.sh

@@ -0,0 +1,39 @@
+source common.sh
+
+if [[ -z $(type -p git) ]]; then
+    echo "Git not installed; skipping flake tests"
+    exit 99
+fi
+
+clearStore
+rm -rf $TEST_HOME/.cache $TEST_HOME/.config
+
+repo=$TEST_ROOT/flake
+
+rm -rf $repo $repo.tmp
+mkdir $repo
+git -C $repo init
+git -C $repo config user.email "foobar@example.com"
+git -C $repo config user.name "Foobar"
+
+cat > $repo/flake.nix <<EOF
+{
+  description = "Bla bla";
+
+  outputs = inputs: rec {
+    packages.$system.foo = import ./simple.nix;
+    defaultPackage.$system = packages.$system.foo;
+  };
+}
+EOF
+
+git -C $repo add flake.nix
+git -C $repo commit -m 'Initial'
+
+cp ./simple.nix $repo/
+nix build $repo |& grep -q "Did you forget to track it in Git" || \
+    fail "Trying to access a non git-tracked file should suggest that it is probably the issue"
+
+rm $repo/simple.nix
+nix build $repo |& (! grep -q "Did you forget to track it in Git") || \
+    fail "Trying to use an absent file shouldn’t suggest to git add it"