nixd: enable autocomplete and hover info in config sets

In NixOS modules options can be defined at the top-level, e.g:

    { foo = true; }

or inside of `config`, e.g:

    { config = { foo = true; }; }

Autocompletion and hover information before only worked for the former
because the option resolving code had no special handling for `config`.

Fixes #566.

Author: not-my-profile

nixd/lib/Controller/Completion.cpp

@@ -280,6 +280,9 @@ void completeAttrPath(const Node &N, const ParentMapAnalysis &PM,
     // Construct request.
     std::string Prefix = Scope.back();
     Scope.pop_back();
+    if (!Scope.empty() && Scope[0] == "config") {
+      Scope.erase(Scope.begin());
+    }
     {
       std::lock_guard _(OptionsLock);
       completeAttrName(Scope, Prefix, Options, Snippets, Items);

nixd/tools/nixd/test/completion/options-in-config.md

@@ -0,0 +1,81 @@
+# RUN: nixd --lit-test \
+# RUN: --nixos-options-expr="{ foo.bar = { _type = \"option\"; }; }" \
+# RUN: < %s | FileCheck %s
+
+<-- initialize(0)
+
+```json
+{
+   "jsonrpc":"2.0",
+   "id":0,
+   "method":"initialize",
+   "params":{
+      "processId":123,
+      "rootPath":"",
+      "capabilities":{
+      },
+      "trace":"off"
+   }
+}
+```
+
+
+<-- textDocument/didOpen
+
+
+```json
+{
+   "jsonrpc":"2.0",
+   "method":"textDocument/didOpen",
+   "params":{
+      "textDocument":{
+         "uri":"file:///completion.nix",
+         "languageId":"nix",
+         "version":1,
+         "text":"{ config = { fo }; }"
+      }
+   }
+}
+```
+
+```json
+{
+    "jsonrpc": "2.0",
+    "id": 1,
+    "method": "textDocument/completion",
+    "params": {
+        "textDocument": {
+            "uri": "file:///completion.nix"
+        },
+        "position": {
+            "line": 0,
+            "character": 15
+        },
+        "context": {
+            "triggerKind": 1
+        }
+    }
+}
+```
+
+```
+     CHECK: "id": 1,
+CHECK-NEXT:  "jsonrpc": "2.0",
+CHECK-NEXT:  "result": {
+CHECK-NEXT:    "isIncomplete": false,
+CHECK-NEXT:    "items": [
+CHECK-NEXT:      {
+CHECK-NEXT:        "data": "",
+CHECK-NEXT:        "detail": "nixos",
+CHECK-NEXT:        "kind": 7,
+CHECK-NEXT:        "label": "foo",
+CHECK-NEXT:        "score": 0
+CHECK-NEXT:      }
+CHECK-NEXT:    ]
+CHECK-NEXT:  }
+```
+
+
+```json
+{"jsonrpc":"2.0","method":"exit"}
+```