fix(dart): mark "direct overridden" pub dependencies as direct

[sueun-dev]

Description

pubspec.lock records four dependency kinds, but the parser only handled three. Pub writes dependency: "direct overridden" for any package pinned through dependency_overrides, and relationship() had no case for it, so those packages were reported as RelationshipUnknown instead of RelationshipDirect.

The kinds come from pub's lockfile writer (lib/src/lock_file.dart in dart-lang/pub):

static const _directMain = 'direct main';
static const _directDev = 'direct dev';
static const _directOverridden = 'direct overridden';
static const _transitive = 'transitive';

direct main, direct dev and direct overridden are all direct dependencies; only transitive is indirect.

This adds direct overridden to the direct case and a regression test (testdata/overridden.lock) with a package pinned via dependency_overrides, which was reported as Unknown before and is Direct after. The stale comment that said there are three kinds is corrected.

Related issues

None; found while reading the pub lockfile format.

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).