feat: notation to refer to no attribute search prefix

An attrPath prefix of "." indicates no need to try default attrPath prefixes. For example 1nixpkgs#legacyPackages.x86_64-linux.ERROR` searches through

```
trying flake output attribute 'packages.x86_64-linux.legacyPackages.x86_64-linux.ERROR'
using cached attrset attribute ''
trying flake output attribute 'legacyPackages.x86_64-linux.legacyPackages.x86_64-linux.ERROR'
using cached attrset attribute 'legacyPackages.x86_64-linux'
trying flake output attribute 'legacyPackages.x86_64-linux.ERROR'
using cached attrset attribute 'legacyPackages.x86_64-linux'
```

And there is no way to specify that one does not want the automatic
search behavior. Now one can specify
`nixpkgs#.legacyPackages.x86_64-linux.ERROR` to only refer to the rooted
attribute path without any default injection of attribute search path or
system.

src/libcmd/installable-flake.cc

@@ -28,6 +28,11 @@ namespace nix {
 std::vector<std::string> InstallableFlake::getActualAttrPaths()
 {
  std::vector<std::string> res;
+ if (attrPaths.size() == 1 && attrPaths.front().starts_with(".")){
+ attrPaths.front().erase(0,1);
+ res.push_back(attrPaths.front());
+ return res;
+ }
 
  for (auto & prefix : prefixes)
  res.push_back(prefix + *attrPaths.begin());

src/libcmd/installables.cc

@@ -301,6 +301,11 @@ void completeFlakeRefWithFragment(
  completionType = ctAttrs;
 
  auto fragment = prefix.substr(hash + 1);
+ std::string prefixRoot = "";
+ if (fragment.starts_with(".")){
+ fragment = fragment.substr(1);
+ prefixRoot = ".";
+ }
  auto flakeRefS = std::string(prefix.substr(0, hash));
  auto flakeRef = parseFlakeRef(expandTilde(flakeRefS), absPath("."));
 
@@ -309,6 +314,9 @@ void completeFlakeRefWithFragment(
 
  auto root = evalCache->getRoot();
 
+ if (prefixRoot == "."){
+ attrPathPrefixes.clear();
+ }
  /* Complete 'fragment' relative to all the
  attrpath prefixes as well as the root of the
  flake. */
@@ -333,7 +341,7 @@ void completeFlakeRefWithFragment(
  auto attrPath2 = (*attr)->getAttrPath(attr2);
  /* Strip the attrpath prefix. */
  attrPath2.erase(attrPath2.begin(), attrPath2.begin() + attrPathPrefix.size());
- completions->add(flakeRefS + "#" + concatStringsSep(".", evalState->symbols.resolve(attrPath2)));
+ completions->add(flakeRefS + "#" + prefixRoot + concatStringsSep(".", evalState->symbols.resolve(attrPath2)));
  }
  }
  }
@@ -344,7 +352,7 @@ void completeFlakeRefWithFragment(
  for (auto & attrPath : defaultFlakeAttrPaths) {
  auto attr = root->findAlongAttrPath(parseAttrPath(*evalState, attrPath));
  if (!attr) continue;
- completions->add(flakeRefS + "#");
+ completions->add(flakeRefS + "#" + prefixRoot);
  }
  }
  }