Support NuGet `dotnet-tools.json` file in the root of the project

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Feature description

Since https://github.com/dependabot/dependabot-core/issues/1697 / https://github.com/dependabot/dependabot-core/pull/7028 local .NET tool files are supported. NET local tool files (`dotnet-tools.json`) can be stored in the root of a dotnet project or in `.config`. Currently only files stored in `.config` are supported. It would be nice if `dotnet-tools.json` files stored in the root of a project were also supported.

Start by locating the existing support for .config/dotnet-tools.json and trace how local tool files are discovered. Compare that path with the project root, then add coverage showing that a root-level dotnet-tools.json is recognized while existing .config support remains intact.

Written by the indexing model from the issue text.